Holley Carburetor: Power Valve Explained

[@novakenss]

I've always run a 650 double pumper Holley on my '66 Nova small block. I run a fairly mild Lunati cam that gives me 19" hg at idle. I use a 7.5 or 8.5 power valve that give me high torque, great throttle response and decent MPG. The only trouble I've ever had is when I rebuilt it last summer and used the wrong base plate to body gasket. The idle circuits, both primary and secondary were blocked off because the gasket holes were in the wrong place. It wouldn't idle and I had to crank the idle screw way up to run on the transfer slots. Using the right gasket fixed that. I learned to look at new gaskets very carefully before using them. You might talk about power valve numbers too. Some are hard to read and figure out I've noticed. Thanks for the video Randy.

[@holleycarbshowtosadvice40y90]

You have to be careful about main body to throttle body gaskets , they can block off idle circuits. Thanks for watching.

[@charlierhiger7478]

Thank you, Randy, I remember changing power valves at the race track and had no idea what it was done or how it works but I do now and when I rebuild my 4412 Holley I will understand that part. Now I just need to learn how to get my vacuum numbers so I can get the right power valve for it. Thank you, I am following you and studying your videos.

[@holleycarbshowtosadvice40y90]

Glad I could help.

[@jackjackson6476]

put a vac gauge on the manifold, start the engine and halve what your reading on the gauge and that where you start on the power valve.

[@doogledog8634]

Great teaching on not just the pv, but on how the whole metering block works. Thanks!

[@holleycarbshowtosadvice40y90]

thanks Eric.

[@hardinelders8457]

Thank you thank you thank you !!!! Finally.... the power valve demystified !!! I had been thinking that vacuum OPENED the valve and nothing made sense. Now that I see that the valve is CLOSED by vacuum at the designated value of the valve I can figure out which way to go. FANTASTIC !!! BEST EXPLANATION EVER !!!

[@davecee4459]

Throttle plates open , lowers manifold vacuum , power valve opens. fuel gauge moves towards the E

[@rodgerburden4230]

Re

[@holleycarbshowtosadvice40y90]

Thanks my friend

[@NeilLB7]

Lol. That last gauge you mention is the one nobody talks about watching until everything seems to be running right. Then that’s the only gauge they ever look at again.

[@StephenGiovi]

I have a new, not Asian made, Holley 650 CFM Manual Secondary carburetor. I had a problem with a rich condition on the AFR Meter-about 12:1. After some time trying to tune the engine, I found that the power valve stuck open. The valve finally closed, and my AFR meter registered 14:1 and it held a steady reading. What I'm saying is that some of the rebuild kits come with some slightly bent power valves which caused some leakage. This condition along with a bad seat is what caused the rich to lean condition. I bought another valve by a good company as well as Holley and found valves in better condition. Look out for performance parts that have assembly problems. Also, watch for your shipping company stepping on your package...end of story!

[@holleycarbshowtosadvice40y90]

I've had a lot of shipping problems with damaged parts, I guess I've been lucky with power valves, thanks for the information, have good one.

[@stevenstreng4788]

Excellent video finally someone who knows what they're talking about

[@holleycarbshowtosadvice40y90]

Thanks

[@Good-Enuff-Garage]

Thank you Randy, with that sweet Southern accent you could tell me anything about automobiles and I would believe it, ha ha

[@holleycarbshowtosadvice40y90]

thanks

[@wrlgmail]

For those without patience to watch to the last few seconds- if you have a ruptured power valve your idle *will* be extremely rich. So an open power valve does not affect idle, a blown valve can happen from a backfire from poor fuel quality and you will notice very poor idle as fuel is sucked around the valve.

[@shawnrenaud2252]

Thanks, man. I missed that detail. I always thought a too high number of power valve (like a 7.5 on a motor with 10 hg vacuum) would pour un-needed fuel at idle. But, if I understand, the wrong power valve will only affect the engine at speed, when stepping lightly on the gas, dipping vacuum under the PV rated number. I have a Ford 390 C6/Fairlane with a stout cam, making 10" to 11" vacuum and a 6.5 power valve installed. It goes great but I've been wondering if I should stick a 4.5 in there.

[@holleycarbshowtosadvice40y90]

Thank you for your help.

[@kentbarger6974]

I'm a retired auto mechanic I have modified the Ford two-barrel carburetor and 2 barrel Holley carburetor for gas mileage my grandfather was head mechanic for a Ford dealership he showed me how to modify carburetors for mileage he also showed me how to modify carburetors 4 high performance I had a 65 Ford Galaxie four-door 289 V8 motor manual 4-speed transmission pulled a 4 by 10 enclosed trailer about 85 to 100 lb tongue weight on the vehicle it was an old motor Depression was about 120 down to 90 lbs compression the compression was low a new motor would have 160 lb of compression I modified the carburetor and got 24 miles to the gallon pulling a trailer for Kansas and Missouri went to Illinois and down to Florida from Utah 23 1/2 miles a gallon up to 24 Mi to the gallon that's pretty good gas mileage pulling a enclosed trailer my grandfather taught me how to modify the carburetors I am a retired auto mechanic

[@holleycarbshowtosadvice40y90]

I'm sure you could start a you tube channel and do well , explaining how to improve fuel mileage .

[@66mustangguymoreau36]

So under normal conditions power valves don't feed the idle circuit unless they are broken. And they help richen up the mixture under load. Different valves are needed depending on engine vaccum. So if your car has a lean condition under acceleration you need to change the jets and maybe a power valve

[@holleycarbshowtosadvice40y90]

the power valve feeds off the fuel in the bowl , the idle circuit feeds from the main well like the jets, but the main well only supplies fuel to the idle fuel restricted other than that they are separate circuits ,unless the power valve diaphragm is leaking then it will pull unmetered fuel in from the vacuum source for the power valve and make your engine run rich , more so at idle, yes you do need to have the right power valve.I hope that helps it can get confusing.

[@geoffernie]

Thanks Randy Could understand why my holley 600 Was running so rich , flooding etc. The cam only pulls 9 inches of vacuum at idle. Power valve is a 6.5 Will try A 4.5. Engine would idle fine , as I spose it's on the primary circuit , bit pig rich just off idle onto secondary circuit.

[@holleycarbshowtosadvice40y90]

Just remember the power valve will not effect the idle circuit richness, unless the power valve diaphram is ruptured or the gasket is leaking, they are two separate circuits. Changing the power valve to a 4.5 with 9.0 inches of idle vacuum is correct , but will not change the idle mixture unless pv is leaking , the pv flows fuel through the boosters only at above 1500 rpms not through the idle circuit . Check out my video on idle circuits mods with high duration camshafts, hope this helps.

[@ford351cleveland]

Greatly underappreciated channel awesome video guys

[@holleycarbshowtosadvice40y90]

Thank you.

[@stephenr1999]

Thank you Sir for passing on this knowledge.

[@holleycarbshowtosadvice40y90]

Your very welcome.

[@1967davethewave]

I've been a professional mechanic for 35 years and I'm really good with carbs, especially the Quadrajet. I learn something useful every time I watch your videos. You sir are an expert! I do have a question about the power valve though. If I want to get a lean cruise for better mpg can I slightly enlarge the power valve feed holes and lean out the main jet size a little? Or is this too dangerous to attempt?

[@holleycarbshowtosadvice40y90]

I think I would try a two stage power valve and lean the main jets , but you can try changing the power valve restrictions and leaning the main jets , just remember it matters what kind of driving your doing , highway or around town.

[@mickangio16]

EXACTLY what you said, Dave. The holes you are talking about are called "power valve channel restrictions" or PVCR. I don't understand why Holley Carburetor doesn't make them replaceable size. Randy's carb videos are the best.

[@trailerparkcryptoking5213]

@@mickangio16 you can drill and tap the PVCR’s and install a brass set screw that you drill out to the size you need. Of course you can drill out a few with different size jets...there is a video on KZbin where a guy shows doing it and what size set screws he used. I believe his channel is called One Car Garage. He has a Barry Grant Holley on a 502 BBC in a ‘63 Tempest....

[@guruguy6800]

Can you please do a video for the Holley 4412 carburetor. How to rebuild a Holley 4412 for high performance and still IMCA legal for dirt car racers. Please..... we have to run a standard 350 engine (it can be 30-60 over bore) with S/R heads, flat tappet cam and lifter and we are required to run a 500 cfm 2 barrel carburetor (4412)...... can you please help me and all the other dirt racers get the best out of our cars.......

[@holleycarbshowtosadvice40y90]

i see what I can do , give me all the information you can , what size track how many rpms your turning , compression , cam and I will do my best , are you limited on rod length.

[@hardinelders8457]

I'm using a Holly 94 carb to power a 1982 Honda Gold Wing GL1100 on a home made intake manifold. The bike starts and runs nice but boggs off the line for about 1-2 seconds , then it takes hold and goes like crazy. the next problem is that at about 60 mph it seems to starve a bit . If I hit the throttle hard it will take off with plenty of power, probably from accelerator pump , but then if I let it settle in at about 60 mph again it has a hard time holding it there.....it wants to slowly drop down to about 55 mph....give it some more throttle and away it goes up to 70 mph but then again if I try to drop back and try to maintain 60 mph it struggles. I've tried 2 different power valves (neither have number stamps so I have no idea what they are) one is the original, probably from a small 49-52 ford dump truck , the other was part of a kit I bought on ebay. The carb's original jets are 52s , right now I'm trying 50s, both have similar symptoms, as do the two different power valves I've tried. If I remember right, idle vacuum is about 18" and running down the highway at about 55 mph on the flat, holding throttle steady , vacuum runs at about 10"-12".....I'm hoping that with your many years experience you might be able to please point me in the right direction as to where to start with jet size and power valve size in this very unique application??? Thank you in advance for any help you can provide.

[@holleycarbshowtosadvice40y90]

Sounds like you might be to lean and work on this first and then work on the accelerator pump. Hope this helps

[@papabapyro]

I have always heard if you have a pop back it blows the power valve. how do you know/what affect does it have? Thank you for the videos, very informational.

[@holleycarbshowtosadvice40y90]

Years ago that was true, but materials have improved alot and power valves have check valves that help protect against a back fire and aren't as much trouble today, befires are bad about damaging brass floats, nitrophil floats are more durable. Now when you rupture a power valve your engine is going to run rich sometimes to the point of the idle mixture screws having no effect on the idle, and fowling the spark plugs. When the pv ruptures the vacuum applied to the pv is now drawing unmetered fuel out of the fuel bowl into the engine. Hope this helps.

[@lunatheshapeshifter4017]

You nail it, mine was blown, changing it now on my Resto mod Clone Yenko Big block Nova

[@holleycarbshowtosadvice40y90]

Great job bud.

[@leenixon9786]

I rebuilt a Holley spreadbore and the only way the engine will start , is with a small pump shot. It will sit there and idle perfectly after started . I can even turn the curb idle screw way down and it will idle down to 300 rpm , but if I turn the engine off then try to restart it , it’ll just keep cranking and cranking until I give it a pump shot . Any ideas??? Thanks

[@holleycarbshowtosadvice40y90]

I don't really understand your problem, your engine needs a little pump shot to start, now if your power valve was leaking i can see it starting without any pump shot from the unmetered fuel. Hope this helps.

[@michaellafreniere5083]

My 600 holley, when I go to take off from a stop it feels like a back fire or a flatspot almost like a Stahl then picks up and goes. I changed power valve,and diaphragm but no change. I stayed with a 6.5. P.v it's a Ford 302 4spd truck and towing cam. Would like a 7.5 help me or would I need to change squirter.

[@holleycarbshowtosadvice40y90]

Mike a lot of things can cause this besides the carb , if its a flat spot in the carb, try increasing the accelerator pump squirter by .006 and make sure and look down the throat of you primaries while moving your throttle open, make sure the the discharge nozzles are squirting fuel as soon as the throttle moves. Also make sue your intial timing is correct and the timing is advancing properly. Let me know if this helps.

[@davidvonanderseck8649]

Man this is an Awsome video. Thanks for your work. I have a 850 vac sec Holly on a 468 bbc with a big cam. I get now the function and transition process. Thanks

[@holleycarbshowtosadvice40y90]

You are most welcome.

[@robkirkland1603]

Help! I keep getting told that the part at the rear of my 1954 Ford Crestline is a spark control valve and NOT a power valve. I'm confused. I have a FoMoCo ECG 2 barrel carb. The part I describe was recently sucking air really bad and the car idles like crap. Any help would be appreciated.

[@holleycarbshowtosadvice40y90]

Rob that was a little before my time, i never got a chance to work on any of them, I know some of the old carburetors had spark control valves that looked like power valves, i believe its a spark control valve but not sure.

[@nickmcminn2137]

Great video. Can anyone tell me why I get fuel in my vacuum advance canister? What would cause fuel to seep from my ported vacuum port? It is a Summit Racing 650 vacuum secondary.

[@holleycarbshowtosadvice40y90]

Nick I've only seen two things causing that , back firing through the carb or the carb flooding after shut off ( hot soak ) condition. Hope this helps.

[@Kmecha84]

If one blocks off the power valve altogether, stepping up 6-8 main jet sizes is enough to keep up from stumbling from gas to go on the hwy?

[@holleycarbshowtosadvice40y90]

I prefer to run a power valve in the primaries to keep control of the fuel curve down low, but if you don't run a p-v probably 8 to 10 sizes up . Would not won't to pay your gas bill , good luck.

[@kevinblackmon6990]

Likely a dumb question. But I don't know. Is there a difference between the primary bowl and the secondary bowl? Meaning can they be swapped?

[@holleycarbshowtosadvice40y90]

No the fuel inlets will be on the wrong side if you switch the fuel bowls, the only exceptions are the bowls that have dual inlets, hope this helps.

[@MarkCarr331]

How can I identify my current power valve if it has no paint or marking on side? I'll tey I can test with vacuum pump and guage to figure it out

[@holleycarbshowtosadvice40y90]

Yes if you have a good vacuum pump you can test it . Power valves are usually marked on the side or on the bottom but some were not stamped, making it easily erased. Hope this helps, take care.

[@Tifford1]

I have a 670 quick fuel carb and have blown 2 power valves. Make my engine (351w, dart block, bored and stroked to 427 cubic inches, 492hp 512 tq) run rich & I've been stranded twice due to a flooded engine. Any suggestions???

[@holleycarbshowtosadvice40y90]

First thing do you run an ignition box and are the spark plugs sooted black , if you could give me any information on your tune like, ignition timing, jetting, does carb have screw in bleeds if so what sizes , main jet size and compression ratio, how much base timing and total timing do you run and also does your carb have a power valve check valve in your throttle plate Do you have to replace your spark plugs cause there burning rich and are sooted black, usually this will make the engine backfire and sometimes a not start. Let me know.

[@shortattentionspangarage1312]

Excellent description of the power valve and how it works. I especially liked your reference to vacuum. Calling it a "high" vacuum or a "low" vacuum is arbitrary based on the person's frame of reference. Is a high vacuum 25 mmHg? Or is it 1 mmHg? I typically use the term "deep" and "shallow" for vacuum reference since to me that's a clearer way to explain how far the vacuum is away from atmospheric pressure (or zero mmHg on a gauge).

[@holleycarbshowtosadvice40y90]

I'm old school Matt lol.

[@ChuckEllis]

This is awesome. Thank you for making this video.

[@holleycarbshowtosadvice40y90]

Thank you for watching.

[@MikeKetchumsGarage]

Will a too small of a power valve effect steady crusing? Cause fluctuations or surging?

[@holleycarbshowtosadvice40y90]

If you are cruising and not under a load ,power valve should be closed, if your jetting is to lean it will surge , and it will open the power valve and all this is assuming the tune is in good shape , no vacuum leaks, ignition timing is correct ect.

[@MikeKetchumsGarage]

Thanks. You confirmed one of my hunches. Gonna try 1 size bigger jet.

[@roaddog1973]

One jet size is hardly anything. You may need to go up two sizes to get rid of the surge. Don't get discouraged if one jet size does not get rid of it.

[@yzrider3326]

So a bad power valve might be the problem with my 1962 Galaxie 500 . It stumbles from a start and will die if I don’t feather it out . Once I’m under way it’s fine . It’s got the OEM 2 barrel 2100 carb .

[@holleycarbshowtosadvice40y90]

Sounds more like the accelerator pump not squirting fuel, should squirt fuel as soon as the throttle opens. Hope this helps.

[@justgriff1295]

Great explanation, just what I needed !

[@holleycarbshowtosadvice40y90]

Thanks for watching.

Can a powervalve fail in the closed position? I am chasing a lean pop condition that I believe to be fuel delivery related.

[@holleycarbshowtosadvice40y90]

Olivier anything possible , but I've never seen it. The power valve usually fails in the rich position, check your power valve circuit to see if the pvcr's restrictors are stopped up.if that don't help , leave me as much information about your problem as you can , and I'll try help you.

[@mikewilliams-mj9rd]

Hi Randy....I want to run two 600 holleys on a supercharger, can I just plug the rear PV and move up 6 jet sizes Thank you

[@holleycarbshowtosadvice40y90]

Yes on a 4150 you can block the secondary power valve and jet up 8 to 10 jet sizes on a blown engine, also you will need to vacuum reference your power valves and may need to lean your idle circuit. Hope this helps.

[@gregballenger916]

I finally understand the powervalve and how it functions,thanks to this video. Great job! Now for my question.... What is the biggest cfm carb thatI can put on a 350 l34 engine w/2.02 valves and a big cam. It's an old vette motor that my father had and i,d like to bring it back to life. I'm looking for barely streetable performance and lots of hp. thanks again.

[@holleycarbshowtosadvice40y90]

Greg if you could send me all your engine information , like what intake manifold [ single or dual plane ] cam specs compression ratio and type of auto trans and converter stall speed , I'll do my best.

[@jimdavidson5208]

You can put a Dominator on it if you want. Might be a little dogy....... A 600 vacuum secondaries is all you need.

[@jakefriesenjake]

@@jimdavidson5208 exactly. Usually the mopar guys are always sayin', "this engine costs me $10 just to start it up..... And, this puppys got dual quads, 950 double pumpers"..... Lol, on a 318! When a proper 500 cfm would give him proper fuel, at the right time. Plus he would get his throttle response back.... I built a 381 Chev. Stroker. Made 511hp and 495 tq. It only has a 650 speed demon on it. A 600 quadrajet probably would be better, due to the smaller primaries. My throttle response is awesome tho!

[@bobepperson1951]

I have a problem with my Holley 4160 vacuum Carburator in my primary Pump Discharge Needle Valve Sticking, no fuel coming out of tne Discharge Squitter until I take off the Squitter and take a small needle nose pliers and wiggle it, put the squittler back on, and everthing works fine.What do I need to do?

[@holleycarbshowtosadvice40y90]

Bob I usually remove the carb and clean the accel pump counter weight cavity with a small bristle brush then install a new stainless steel counter weight in the pump passage and use a punch and a hammer to lightly hit the top flat end of the counter weight to reseat the counter weight in the pump passage seat , this renews the sealing point and usually with both cleaning and reseating will cure the problem, make sure the area around the seat is clean, hope this helps.

[@jimsoderberg1200]

Randy will I see fuel leaking,, looking down the throtle body to the manifold With a blown power valve ?? thanks ,, Great Vid .

[@holleycarbshowtosadvice40y90]

Jim sometimes you will see fog coming out of the venturi area if the engine is up to temp. Where the power valve circuit is you can't see .It matters if the diaphram is completely ruptured. This will drain most of the fuel out of the bowl and the engine will turn over excessively to start, and run very rich .

[@paulbedford9816]

Randy I love your films. I have an Olds 350 in a Cadillac Seville, the EFI is gone, now a Holley 4150. Suddenly the car is eating fuel? I've been doing a lot of highway travelling between 65 and 75mph (France) and she's eating fuel. She does smell rich. May I ask if you have any immediate thoughts on what might be wrong please?

[@holleycarbshowtosadvice40y90]

If it was fine and suddenly started using more fuel I would look at the power valve (leaking diaphragm) first and correct, or possible float leaking or damaged float from a backfire, and if you run a distributor with a vacuum advance and diaphragm ruptures it also cost fuel mileage, this is where I would start. Hope this helps.

[@paulbedford9816]

Thanks so much, terrific insight. I'd better get to work. Paul@@holleycarbshowtosadvice40y90

[@geerbangr]

Kick ass video Randy, thanks!!

[@holleycarbshowtosadvice40y90]

Thank you.

[@hot351xy8]

Hi Randy i have a Vacuum reading at idle in gear of 3 The Power valve in the Carburettor at present is 4.5 Do i need to put a smaller Power Valve in Thanks Stratos

[@blubu68]

A better way to check the PV is take the vacuum reading at cruise and go a couple numbers below that.

[@holleycarbshowtosadvice40y90]

You could go to a 3.5 pv but I would first recommend checking your ignition timing and I take it you have a big cam I would try recurving my distributor and bring my initial ignition timing up as high as i can that will raise your engine vacuum or if I was using the car for just racing I would lock out the ignition timing in the distributor. Hope this helps

[@steveharrison4705]

Another fantastic video, keep them coming, guess a blown power valve can be caused by a violent backfire so I was told, I'm just inches from removing my almost new aluminum double pumper ultra series carb from my 363 stroker ( only 250 miles and 2nd set of plugs since carb was installed), I did have a few backfires thru the exhaust and maybe 1 or 2 blow back thru carb, but was told new carbs have a power valve protection from being ruined is this so? Also my accelerator pump arm at the diaphragm housing leaks when activated which I know is a problem, I really thought holley were a little more tougher than what I've experienced so far especially right out of the box.

[@holleycarbshowtosadvice40y90]

Most new carbs have backfire protection but sometimes they will damage floats and even nitrophil floats will bend there arms check your floats for binding. Also if your leaking at the pump diaphragm check first by looking down your primary accelerator pump aquirter and watch as you move the throttle do they squirt fuel the instant you move the throttle if they don't squirt remove the carb and remove the accelerator pump squirters one philips head screw and wiggle the check valve till it losses up, turn carb upside down and check valve should fall out clean and lube with wd spray a nd also check pump diaphragm for loose screws or possible bad diaphragm. Hope this helps.

[@robpeters5204]

A lot of guys say to put 2 gaskets on the power valve. Is this a good idea? Great explanation!

[@holleycarbshowtosadvice40y90]

No , just one gasket.

[@baims8367]

What would u run for power valve and acceleration pump cc. On a gm 602 w Holley 500 used for circle track racing 6100 rpm. I’m a bit lean carb came w engine aboit to open it up tonight tho. I was thinking 6.5 Power valve. And 50cc accelerator pump and maybe jump 2 jet sizes up ?? 435/460 lift came in these 602 crates 6100 rpm thanks 🙏🏻 😊

[@holleycarbshowtosadvice40y90]

Sounds like a good plan.

[@drewdowncutomz4792]

Have any videos of leaks of gas on top of carb

[@holleycarbshowtosadvice40y90]

Drew I thought I had one , I will do my best to have one out in the next few days. Make sure you test your fuel pressure, even if it has a stock mechanical pump. On a street car without a regulator should be under 7 psi , watch for vid.

[@songohn]

Hi Randy. I enjoy your teaching style and your accent. I'm just in the middle of a carb clean up for a 57 T-Bird... 1273-1 Holly. First attempt at doing this. There are four .096 holes drilled in the body of this thing that I don't understand and I know you can help. One hole per bore about half inch above the throttle body surface, between two webs cast into the exterior of the each "barrel". Clearly these hole would be drawing air, and dirt, from the outside of the carb. Could this be a mod of some sort? It makes no sense to me. Have you seen or heard of this before? Should I drill to size and tap for a set screw or something? My curiosity has put a halt on the reassembly. Please advise. Thank you!

[@holleycarbshowtosadvice40y90]

I really cant remember ,maybe shoot me a picture of your carb , are the holes above the butterflies and are they seen externally

[@pepitomateo1]

Hello, I’m running a holley avenger 870 it’s rich at cruise and lean at full throttle, any recommendations please

[@holleycarbshowtosadvice40y90]

Holley 870cfm vacuum secondaries non changeable air bleeds , check for bad power valve or wrong power valve , check jets for proper size if you are rich at cruise then jet down and may have to jet up the secondaries if the secondaries are lean. Now this is if the carburetor is clean and in good shape and clean passages. Make sure fuel pressure and volume are adequate.

[@ghostrider8913]

Need help I have a 750 double pumper can't adjust the idle screw mixture when I go in with the idle scwew mixture my rpm goes up they say it's a vacuum leak but there is none can you help me with this problem thanks

[@holleycarbshowtosadvice40y90]

First check to see if your idle air bleeds are stoped up that will cause the idle circuit to be rich , when you screw in your idle mixture screws and the idle speed climbs this is a rich condition , again check your idle air bleeds , looking down the carb throat they are the two outer bleeds. Let me know if this works , also look at your transfer slots to make sure they are not over exposed, also make sure your secondaries are adjusted properly or not hanging open, one last thing check your float levels.

[@gusswalker]

Hello, I have a 2300 2bbl Holley Carb and since i bought it (used) ran pretty good until a few months have a serious problems with fuel when taking hills, carb leaks thru the boosters like when inclinates. I changed the power valve twice, adjusted the float, blow up the purges on main body, check gaskets and everything but still getting the big leak. My question is Do I use a Block-off plug ?? Im runing with 73 Jets. Thanks for your attetion and hope you answer.

[@holleycarbshowtosadvice40y90]

Check fuel pressure first should be around 5 to 6psi , no more than 6psi , next if you put air pressure through the carb vents you could easily damage the float . Hope this helps. Ps stock fuel pumps go bad and will easily pump 10psi or more.

[@rebelrobz]

Took my carb off. Holley Street HP 750 Vac Adv. It was surging at cruise. Cleaned really good with spray and put new primary and secondary gaskets on. Also put a new power valve gasket. When I took it off the gasket split. Put the new one on it was thicker. Bolted carb back to car and damn thing will not start. Did not touch anything else but new gaskets. Getting fuel and spark. Chevy with an MSD. Took it back off to inspect and I do not see anything wrong. Weird. It always idles and runs great.

[@holleycarbshowtosadvice40y90]

check to see if carb is flooding or float level is to high , next check the accelerator pump and make sure it squirts with throttle movement . a lot of carb kits comes with gaskets that fits several different carbs , sometimes the wrong gasket will keep the accelerator pump from working ,hope this helps you.randy

[@kato1140]

Thanks Randy !!!! Your vids have helped me greatly !!!!!

[@holleycarbshowtosadvice40y90]

You are very welcome.

[@potshot23]

Still confused, my vacuum at idle in gear (automatic) is 5-6. I wanted to instal a 2.5, but wondered if that would run too rich on the street, so I went with a 3.5, what do you think. My car is street/strip, and has a high overlap cam.

[@blubu68]

Get a vacuum gage and tape it to your windshield, drive the car in 3rd gear at around 3000 rpm record your vacuum, than accelerate half throttle and see how low the vacuum goes, install a power valve to that number. It helps if you have a A/F monitor. My 496 BBC has the same vacuum as you do, I run a 750 on the street with 70 jet in the primary 82 in secondaries and a 9.5 power valve, it idles a 13.5 a/f my cruise a/f is 13.5 to 14 when I accelerate it goes to 12.5, its better than fuel injection.

[@jimdavidson5208]

Yup a wide band O2 will help you dial in a carb nicely.

[@holleycarbshowtosadvice40y90]

I would run a 4.5 p.v. at idle the power valve does not effect idle at all open or closed unless the diaphragm is leaking but 4.5 should be fine. Hope this helps.

[@tonyamatucci1999]

I have a 750 DP race carb. Just got an AFR gauge. It’s pretty good at idle and cruise (12.3-13.7) , but when I mash the gas, it will sometimes go in to the 16’s, briefly. I’ve messed with pump cams, squirters, bleeds. I have it pretty close. Not sure what I should go to next. Main jets, PV, PVCRs. I have a 3.5 PV now. 5hg at idle in drive, but about 10-12 when staging or cruising at 40. Thinking to try going back to 6.5 PV, or higher, so it will open sooner. What do you think.

[@holleycarbshowtosadvice40y90]

When it goes in to the 16s does it hesitate or stumble briefly?

[@tonyamatucci1999]

@@holleycarbshowtosadvice40y90 no. It seems really good. It did prior to me doing what I did. It was going up to 20-21. And it is real brief. 16, then like 13.6 under WOT. Am I being too picky, if it running good?

[@holleycarbshowtosadvice40y90]

I think you have a great afr sensor to pick that up that fast, if your don't have a drive ability problem your chasing ghosts lol.

[@dwightledlow3261]

I burnt a head gasket a couple mounts ago and last week I burnt a piston on a 355 with vortex heads a couple people told me I'm running lean.at a 120/130 mph it a 650 holly do I need to install bigger jets.

[@holleycarbshowtosadvice40y90]

I need your information on your motor and fuel system carburetor size and jets any mods , total timing ,compression ,power adders, which piston burnt #5 or #7? , and water temp. which firing order are you using, camshaft determines firing order , all cam specs ,fuel octane level any info you can think of etc...

[@trlprk]

Randy, I have a stock 307 Chevy with a Holley street avenger carb. I get a hesitation when I transition from cruising to more throttle. I have a 6.5 power valve in the carb, should I change it out for something higher? At idle, in gear the engine pulls 18/19 inches of vacuum. My initial timing is 12 degrees, total is about 32

[@holleycarbshowtosadvice40y90]

Those old 307s were good motors , sounds like you have a lean condition, I don't know the size of the carb you have but those motors liked ignition timing usually 36 to 38 degrees total , as long as no spark knock. If you can drive it with a vacuum Guage then you can watch and see when it's hesitating , may have to make an adjustment to the power valve opening or power valve circuit , there is other things that can cause this condition .Hope this helps.

[@stevenbeck7054]

take a shot every time he says power valve

[@dougwilliams44]

Oh we do! 😉🇺🇸

[@holleycarbshowtosadvice40y90]

Thanks for watching

[@gavmorgan1846]

Hi Randy I like so many really appreciate you sharing your knowledge and now have a better understanding of the holley carby. I was wondering how would you restrict those orifices from the power valve? I am building a 2ltr ford pinto motor bored and stroked to 2.2ltr and wanted to run a 4412 500cfm . I've read in a David Vizard book that for this application to restrict these orifices to around 17 thou . Any other tip would be appreciated too. Also do you think you will be doing anything with a 2 barrel in the future? Gavan. Australia.

[@holleycarbshowtosadvice40y90]

Built several of these carbs years a go , I would start out with .018 on the power valve circuit restrictions and 56 main jets , and drill your air bypass holes in the throttle plates to .150 , I believe I ran a 4.0 power valve and I think I changed the accel pump to a 30 cc diaphragm and housing and pink colored cam, I think I was running .020 accel pump discharge nozzles ,then work on your tune from there, this is the best I remember. Hope this helps.

[@MattsRageFitGarage]

Got a PVCR tuning question for you. I have an 1850-5 on my 390 in my galaxie and my idle, transfer slot and cruise afr is 13.5-14.2 afr depending on situation but when the power valve opens up it goes from say 14:1 average afr to 11.5-12 depending on load, this is light to mid throttle acceleration either taking off from a stop or just stepping on it a bit in high gear, not enough to even consider opening the secondaries. What is the ideal afr with the power valve open? 12.5-13? I take it that I need to reduce the pvcr size a little, probably by about .004"? Also, I'm running 67 primary jets when it came with 66 jets originally. What do you suppose the ideal cruise afr is with an all iron 10:1 390 FE with a 218/224 duration @.050 cam with .513/.519 lift? seems happy at 13.5-14 depending on speed and load and the mpg isn't a bit different than with the stock 66 jets in it. I think there is more mpg potential to be had when the power valve is open and not dumping excessive amounts of fuel?

[@holleycarbshowtosadvice40y90]

Your afr numbers sound good as long as your mixture goes back to a good lean cruise. I don't know if you pull heavy loads, but don't go to lean. You can try a two stage power valve that opens at different rates, its a good idea to use a vacuum gauge with your afr gauge to monitor power valve opening or lean or rich spots. You can try playing with pvcrs or power valve opening, the two stage pv might be watch your looking for. Hope this helps.

[@MattsRageFitGarage]

@@holleycarbshowtosadvice40y90 Something definitely needs done with the power valve circuit. I do feel I would like it to open slightly sooner reading the gauge. It has a 6.5 in it now, maybe 7.5 would do it. The mixture when tipping in the throttle will sometimes lean out into the 15 range before the power valve opens up b you can feel the engine get kind of lazy when the power valve opens up, so it definitely needs leaned out a tad. I did a 45 mph cruise to WOT pull tonight and it went to about 11.5 and blew a bunch of black smoke when the secondaries opened. I converted it to secondary metering block and put I think 71 jets in it. I got it pretty dang close guessing at it, I just installed the AFR gauge the other day. I did get some pinging in the midrange and WOT tonight, so I need to delay the centrifugal advance a little, it's 36 degrees all in at 2700, think I'll shoot for all in at 3000 since it's picky with fuel quality. Sure is fun when it slams ya back in the seat and takes off though!

[@arcorob]

Great video ! Help me understand something...So if I want the power valve to open SOONER I get a higher number ? I ask because my Holley 670 has a 6.5 - When I am cruising (say 40mph) and goose the gas, I get a momentary stumble and I see my AFR gauge go lean for 1/2 second...then its bat out of hell. So should I move to perhaps an 7.5 or 8 ? With my engine and cam, I pull about 13 vacuum (thats in park so I have to see in D) - so just to summarize, higher number, sooner flow , lower number later when vacuum drops?

[@holleycarbshowtosadvice40y90]

Yes a higher number on the power valve means it opens quickly a lower # opens later . If that don't take care of your stumble let me know.

[@arcorob]

@@holleycarbshowtosadvice40y90 Thank you ! The engine runs pretty awesome (347 Stroker with mild cam, blueprinted, aluminum heads) She is a screamer but that dang lag drives me nuts ...its split second but even the AFR gauge confirms, for that split second, she goes lean ...then BOOM ...torque monster :-)

[@holleycarbshowtosadvice40y90]

Robert your carb does not have much adjustability with your street 670, a brawler 680 cfm part# 67317 would be a better choice , with that said you could have your secondaries coming in to soon , or you may need to jet up , I don't know your air fuel ratio , or you might need a longer acc. pump duration and may help to go to a bigger pump discharge nozzle, lots of different things to look at.hope this helps.

[@michaelerdman3330]

This was an excellent tutorial.

[@holleycarbshowtosadvice40y90]

Thank you

[@landondc4739]

I have an FE motor that will idle great and run like crazy. I’ve checked everything I can think of. However, when I get this engine to idle nice, I blip the throttle and then it idles like garbage with lots of liquid coming out of exhaust. Immediately turn it off and restart, idles perfect, no liquid. . Blip again and it idles like shit, turn off, restart, good again. Hammer the throttle and let off and it will idle nice again. Am I having a power valve issue? I was thinking weak mechanical advance springs that don’t want to return home?

[@holleycarbshowtosadvice40y90]

Need more information like carb list # , type ignition , power brake booster? anything else you can think of. Power valve will not effect idle unless diaphragm is ruptured.

[@landondc4739]

List #9834, converted elect ign, power brakes, two vacuum gauges report 15-20 psi, electric fuel pump with about 4-6psi and good volume to carb. I have noticed is that when it starts idling terrible, the vacuum goes way down and jumps around a lot. Also when idling nice timing is about 8deg and when it goes to shit the timing is 11ish deg. Plugs look good, replaced cap. I really appreciate any info you have, this one is driving me crazy. The power valve was something I didn’t consider so was wondering if that might be my issue.

[@holleycarbshowtosadvice40y90]

I don't believe the power valve is bad, should be running rich with black smoke all the time. The power valve only effects idle if the diaphragm is ruptured. The liquid coming out of the exhaust is fuel or water? can you explain ,when the rough idle happens is exhaust rich or lean. I'm still thinking hang with me. By the way are you running a pcv valve?

[@landoncarpenter4140]

@@holleycarbshowtosadvice40y90 No pcv valve. I ended up taking the distributor all the way down and it all looks good. Decided to tear into the carb and as i was pulling the metering block off, what I noticed as I was ripping the gasket apart was a piece of crap in the "power fuel passage" on the fuel inlet side of the metering block. Bowl was clean, jets clean, fuel inlet screen clean (although deformed a bit as if it didn't seat right in the fitting), mix screws clean. Going to get a gasket set and put this thing back together, hoping I found the issue.

[@tripleh9985]

You make awesome videos. Very informative.

[@holleycarbshowtosadvice40y90]

Thank you

[@luizfernandopaquieli3709]

Hi Randy, I'm watching your movies to adjust my Holley , but I've seen yet nothing related my carburator issue. I have a Holley 4160 600 cfm vacuum secondary installed on a Ford V8 302, when I started to throttle the nozzle is injecting perfectly, however it seems that the fuel isn't being puxed by the vacuum through the vents. When I cover the carburator a bit with my hand and keep throttling the acceleration clean. Do you know what can I do to adjust that?

[@holleycarbshowtosadvice40y90]

I not sure , but if you cover the carburetor partially with your hand a it runs better , you have a lean condition, could be partially stoped up like the main jets or float level way low, wrong jets , or a bad vacuum leak , lots of things can cause this concern , might just need an overhaul, I'll try to do a video before long.

[@holleycarbshowtosadvice40y90]

Could be anything from a vacuum leak to a block fuel circuit , if you are having to partially cover the carb to get it to accelerate , that sounds like the jets may be restricted or not the right size .

[@luizfernandopaquieli3709]

@@holleycarbshowtosadvice40y90 I've fixed this issue, It was a minor gasket leak and pcv failure! Those failures combined was causing that issue. But now I'm facing a new issue, when It starts throttleling happens some holes like cutting fuel. I've adjusted the injection pump arm but nothing change. I'm thinking the carb is quite big to the engine. It's a 650cfm to a Ford 302. What do you think?

[@joe-hp4nk]

What size power valve for the secondary's?

[@holleycarbshowtosadvice40y90]

Joe if you run a secondary pv, I would use the same size as the primary, I block my secondary power valve and jet up 8 to 10 sizes on the sec. mettering block

[@TheThirdWheel618]

I wanted to ask you do you ajust your idle air speed with the center screw that is under the air cleaner stud ? I never knew one was there!!! I always ajust my idle by the pump cam screw I watched a old tech video from the 50"s and they showed that screw under the air clear stud it said to ajust air idle speed to 500 using that screw ?

[@holleycarbshowtosadvice40y90]

Some holleys and Barry grants have a adjustable air bypass under the carb stud , this is the same as drilling the butterflies but better, set your carb butterflies with your idle screw to get your transfer slot to about .030 and reinstall carb and set your idle bypass screw under your stud to the correct idle.

[@kevinwilliamson1426]

when blow-thru turbocharging, is it absolutely necessary to boost reference? thanks for your videos..

[@randallcribb7015]

Tom I don't know much about blow through carburetors ,sorry wish I could help , thanks for watching my videos, randy

[@TutorialTelevision]

Tom, I realize this is coming a little late, but the answer is YES. If you don't boost reference, you will have unreliable fuel flow. For example: if you are running 7lbs of fuel pressure and 7lbs of boost, your fuel pressure will be 0. Having a boost reference will adjust your fuel flow as the boost builds/decreases.

[@holleycarbshowtosadvice40y90]

I don't do forced induction. Sorry

[@raymondmoreno7442]

Thank you sir great video

[@holleycarbshowtosadvice40y90]

Glad I could help.

[@thejohnboatfaithfishingand8078]

So where does it get its vacuum signal from? (Or manifold pressure if you will)

[@holleycarbshowtosadvice40y90]

Behind the power valve in the main body there's a cavity ,.at the bottom of the cavity you will see a small vacuum hole .

[@thejohnboatfaithfishingand8078]

@@holleycarbshowtosadvice40y90 Thanks. Trying to help someone on Team Chevelle figure out why he's having an intermittent issue with the power valve NOT opening when he goes to wide open throttle.

[@ll6946]

My idle is set to about 1000rpm. in gear its about 650. 350sbc with a huge cam. Idle gets really low at stoplights sometimes 400rpm any advice? Is it my powervalve being too big or small? I thought floatbowls might be set too high but its the same even if i clear it out. Currently reading the holly carb book from 1980 my dad just gave me lol

[@holleycarbshowtosadvice40y90]

Need to know vacuum reading in gear , and is it a automatic trans and how much stall or is it a manual trans, if you can give me all the specs of motor , compression , duration at .050 and intake centerline, I'll sure try to help you.

[@williamianiri9861]

Very informative video thank you. I have a quick question my Holley 950cfm 4150 carburetor did not have a power valve in the rear from holley.it is a street driven standard shift car. Around town it's a little finicky. Would adding a pv to the rear meter block help? Ty

[@65sohc]

Since secondaries are not open in city driving, unless you're street racing, a secondary power valve would have no effect. 950 is a huge carb. Is your engine producing enough power to warrant that large a carb?

[@holleycarbshowtosadvice40y90]

Not really.

[@gashiflett1]

Randy, I have a 1.9 isuzu carb. The rebuild kit states "no power valve" included. Do all carbs have power valves ?

[@holleycarbshowtosadvice40y90]

Most carbs due, some have a vacuum operated pv diaphragm some have vacuum operated metering rod piston there are exceptions like a ford variable venturi carb, some have pv that are not usually replaced there all metal and don't require replacement, unless damaged. Hope this helps.

[@hawaiidad]

Can you please tell me what is causing my primary idle screw to run best at 1 1/2 turns out and the secondary idle screw runs best at 3/4 turn out? What do I need to change in the secondary. I have an Ultra Double Pumper 650 on a 318 Dodge...Thanks

[@holleycarbshowtosadvice40y90]

first make sure all idle bleeds are the same size , that's the outer air bleeds front and rear , and idle fuel restrictions are all same size , now here is the tricky part , you have to have the idle set screws primary and secondary as close as possible being even, then set your mixtures screws run them all the way in and turn them out around one to one and a quarter turns out , warm engine then make same adjustments to front and rear keeping them even ,hope this, r helps, don't worry if they don't match perfectly, that well be close randy

[@randygenet1320]

Holley Carbs: How To's & Advice ,40 yrs of exp.

[@cefb8923]

Thanks for the video, very helpful.

[@holleycarbshowtosadvice40y90]

Thank you

[@codymorgan6839]

Got a question my 4160 sucks air were the vacuum advance at idle when its should suck when you get throttle any thoughts

[@jimdavidson5208]

You have a gasket missing or leaking. Vacuum sucking is just that. The vacuum secondary canister will have manifold vacuum all the time. Throttle depending on how much of course. Then create your scenario accordingly.

[@holleycarbshowtosadvice40y90]

Were is your hose at on your carb should be on the vacuum tube on you metering block or on after market throttle plates you'll have to have it on ported vacuum. Hope this helps

[@BTOURES]

Hi Randy Great videos. I have a question. I have a brand new Street Avenger Ultra 750 CFM installed on a brand new crate motor. I also installed a A/F gauge as well. My problem is that its runs rich at idle. I want to adjust the primary so that its not into the transition slot, but instead of drilling the holes in the butterflies, can I just open the secondary's a bit for the extra airflow?

[@holleycarbshowtosadvice40y90]

you can up to a point , i prefere my way, or buy a holley ultra with a designed in idle air adjustable bypass. i believe some barry grant carbs had this too.

[@jimdavidson5208]

Set your idle like 600 rpm (small cams of course). Again set idle at the LOWEST you can keep it running. Then tune the mixture screws in the front bowl. Keeping them the same number of turns out. Shouldn't be more than a full turn OUT from GENTLY seated in or closed. Screwing in the mixture screws closes the fuel to the idle circuit. Should darn near kill the motor. UNLESS your power valve is dead or having issues. Also lower the main jet 2-4 sizes.

[@TheCanadianBubba]

Thank you for this 👍

[@holleycarbshowtosadvice40y90]

Your very welcome and thanks for watching.

[@Speed_Shed]

Great video! Now could you tell me what power valve and jet sizes i need for a completely stock 1968 Ford 429 with a BG 750? It runs pig rich no matter how i set it...

[@craphittingthefan2360]

did you not listen to him check your vacuum and go half of that in a power valve, I think you have a float problem.

[@Speed_Shed]

Crap Hitting The Fan Yea, I heard him over a year ago. You living under a rock?

[@holleycarbshowtosadvice40y90]

A lot of these comments I never saw back 3, 4 years ago there was a problem with the channel I'm sorry.

[@hawaiidad]

Ok, how do I determine the secondary power valve size? I have 3.5 in the primary since my vacuum at idle in gear is 7. Thanks

[@holleycarbshowtosadvice40y90]

the secondary power valve is not as critical as the primary , most of the time I run 3.5 to a 6.5 , remember one thing even the primary power valve setting at half the engine vacuum is not set in stone ,I might put a 5.5 power valve in the primary side , and a 6,5 in the rear with 7 inches of idle vacuum, its all in what you are using it for and the fuel curve , if looking for performance and fuel mileage running the power valve at half the engine idle vacuum is the way , hope this helps randy

[@randygenet1320]

Holley Carbs: How To's & Advice ,40 yrs of exp.

[@davidrowland2354]

So if I have 14" vacuum in gear that means I need a 6.5 rite.. I been using A 8.5 AND IDLING RICH

[@holleycarbshowtosadvice40y90]

Yes a 6.5 will work fine but remember the power valve circuit is separate from the idle circuit , so unless the power valve diaphragm is ruptured it will not effect the idle. Hope this helps.

[@arcticcatmatt]

I had a 10.5 powervalve in my blower carb. I finally measured vac at idle and it was 14-15. So, I just put a 7.5 in now.. what side affects would the 10.5 have given me

[@jimdavidson5208]

Early rich fuel mixture in the fuel curve.

[@holleycarbshowtosadvice40y90]

Where are you referencing your vacuum to the power valve from?

[@matthewmarsh1971]

@@holleycarbshowtosadvice40y90 Straight from intake.. I need to see someone elses setup to figure out if I have my car plumbed right. I'm a 'rookie' trying to learn and lots of confusing resources out there.

[@socalsaunalifestyle]

Question? Am I correct is saying that if the rating of the Power Valve is lets say a 6.5 and that's higher than the idle vacuum at 5 inches of vacuum at 900rpm the engine will run too rich because the Power Valve is Open, and if the Power Valve is a 2.5 the engine could run lean until the engine vacuum is below 2.5 inches of vacuum? On this new build I have a new AED 750dp on a Chevy 302ci with 10.5 compression, Comp Cams custom grind solid flat tappet with .516 intake and .537 exh lift with 242 intake and 250 duration @ .050 and with a 110 lobe separation, valves set to .012 cold, new MSD ignition and pro-billet distributer, Victor Jr intake, AFR 180 Street Heads bolted to 1-5/8" headers to 3" flowmasters. It mated to a M21 trans and 3.90 gears on a 28" tall tire. I had the timing at 19* initial and 36* with the same problem but my Brother told me it was too much for 91 pump gas so I lowered it to 13* btdc initial at idle and 32* total. Plugs are all consistant light tan color as well. The problem I'm having is an intermittent lean pop and low power through the carburetor when under load accelerating in 1st gear under 3k rpm and inconsistent idle rpm when warm to operating temperature. I'm at 1-1/2 turns out on the Air/Fuel screws, accelerator pumps adjusted properly with cams in the #2 position. I just opened the secondary butterflies because too much of the idle slot was showing from the underside of the primary butterflies (this did help the idle but is lower idle at operating temperature than when warm). The standard squirters are 31's as well. The vacuum at 900 rpm is at 5 inches so I put a 2.5 power valve in it and didn't think about it much until I watched your videos on this subject. I'm thinking I should have a 4.5 power valve. Am I on to something here or what else can I check or adjust? BTW, above 4k rpm it's an ANIMAL for a 302! Thoughts?

[@holleycarbshowtosadvice40y90]

The power valve will not effect the idle circuit, the power valve only flows fuel through the boosters and they usually start flowing fuel around 2000 rpms, now if the power valve diaphragm is ruptured it will make the idle rich and thats very possible with engine backfires and also check your floats backfires will damage them too. I always try to run my initial timing as high as I can usually around 15 and total timing about 34 to 36. If you still have problems check your cranking compression and let me know.hope this helps.

[@socalsaunalifestyle]

@@holleycarbshowtosadvice40y90 Thank you for your response! That helps!

[@holleycarbshowtosadvice40y90]

Anytime

[@steveharrison4705]

BTW my carb has run rich since the day it was installed and tuned from my engine builder!

[@jimdavidson5208]

Get a bus pass or take Uber if you can't tune a Holley. Engine builders test on a stand or dyno. Which BOTH methods are waaaay off from real street/track driving which equates to engine loading. Everyday the weather changes, that affects mixture.

[@holleycarbshowtosadvice40y90]

I think if you watch my videos you can straightened out this concern. If not let me know.

[@Dodgechallengersixpack]

Thank you sir. Im digging in and trying to educate myself on carbs. Any advice on 6 pack carbs?

[@holleycarbshowtosadvice40y90]

Most 6 packs idle off your center carb get it set up right to start with, and a trick for the front and rear carbs was to run a vacuum hose from one diaphragm to the other like a balance tube , if yours does not have a balance tube you can modify the top of your diaphragm housing if you have the room , this helps to bring in the front and back carbs together, some of the tri power set ups came this way some did not.

[@Dodgechallengersixpack]

Holley Carbs: How To's & Advice ,40 yrs of exp. I see what you mean. They share a vacuum tube to ensure they open up at the same time.

[@jamesrobinson9062]

What is most common power valve for street carb

[@holleycarbshowtosadvice40y90]

Most common power valve is probably a 6.5. But again that can change with a racing cam, when the engine vacuum becomes very low, you have to compensate , or maybe consider a 3 circuit holley .

[@marcstlaurent3719]

Thanks Randy , nice refresher

[@holleycarbshowtosadvice40y90]

Thank you for watching

[@aaronburleson2454]

Quick fuel 750 slayer will I need to change the power valve to say a 5.5

[@aaronburleson2454]

Chevrolet 383 stroker

[@holleycarbshowtosadvice40y90]

If you have good vacuum at idle ,I don't know why you would want to put a 5.5 power valve in it, when you have 19 inches of vacuum at idle. I would like to know the specs on that cam, because I've never seen even a mild camshaft have 19 inches of vacuum at idle.

[@marcfike3701]

Randy what would cause my carb to increase idle speed as the engine temp increases?

[@jimdavidson5208]

fuel mixture or something sealing (metal expanding) closing a vacuum leak??

[@holleycarbshowtosadvice40y90]

Thats natural to gain idle speed with heat , only other thing is if your carb has a hot idle compensator valve that can raise the idle , when the engine is hot. Hope this helps.

[@nelmajeanhix]

I HAVE A HOLLY TWO BARREL HAVE HAD TWO OF THEM ON MY 302 THEY BOTH DO THE SAME THING ,AS LONG AS THE ACCELERATER PUMP IS PUMPING FUEL ITRUNS AS I PUSH THE GAS PEDEL IT ACTS LIKE THERES NO FUEL THEN SLOWLY IT WILL START TO RACE UP IDLE IS PERFACT ANY IDEAS?. THANKS

[@boingkster]

Could be an accelerator pump diaphragm problem, or maybe you just need a larger shot for the pump. A lower number power valve may also help.

[@holleycarbshowtosadvice40y90]

first check for vacunm leaks around carburetor ,next check fuel flow to the carb and fuel filter for restriction, if these are good check for vacunm leaks to brake booster and check at the vacunm tree at the rear of the engine where all the vacunm lines feed out of the intake manifold [ block all them off, if not must be carb stoped up , sounds like a vacunm leak to me, hope this helps randy

[@holleycarbshowtosadvice40y90]

could you explain to me again about your holley 2 barrel , not sure if i understand,randy

[@fuzziepandas]

im having the same issue on my 12a rotary rx7

[@mortenarra4233]

Thanks for the video! I do have a problem with performence and idle on my 289. Shiftet from edelbrock to holley and the IDLE problem seems to still be there on both carburators. But with the holley the PERFORMANCE issue is much more easy to define. The vacuum at idle is at 18 / 19 (Depends on tuning). The symptom is: When i hit the trottle and hold the trottle steady it accelerate very poorly for up to 5 sekund then suddenly it takes off and runs much more smootly. And now it also runs much better then the Edelbrock. Any thoughts? Im gonne connect a vacuum gauge and observe it when I drive to see if it is on a particular value the issue goes away. (I have checked timing degrees and run it with and without vacuum advance, the issue is always there.)

[@jimdavidson5208]

Sounds like a low rated power valve coming in too early. Try a 9.0 or 9.5 power valve. Make sure you check your second paragraph at FULL manifold vacuum.

[@MCFABRICATION]

Did you ever get this figured out? I’m having the same issue with my 289… I’m trying to figure out what power valve size and what jet sizes I should put in on a rebuild I’m going to attempt on a QFT Slayer 600 V.S in a couple days. I’m in Southern California…

[@mortenarra4233]

@@MCFABRICATION No sorry, didnt fix it, but change back to the Edelbrock 500. The bog is gone, but still not so much power at low end as I would wanted, but runs very good in middle and up. I can live with that:)

[@65sohc]

@@jimdavidson5208 Actually the lower the pv rating the later it comes in.

[@holleycarbshowtosadvice40y90]

You need to know how much ignition timing your getting total timing.

[@PiliaeAl]

I was wondering why my cleveland has lower vacuum at idle than stadard- its cammed- now I know how the power vslve works i can tune my holley;

[@holleycarbshowtosadvice40y90]

If your cam is to big your engine vacuum will drop , you have to match everything , if the cam has a late intake valve closing and a lot of overlap it will drop your static compression down dynamically to much, and will lower ldle vacuum and hurt idle quality and low end performance there is a way to help regain some perf. By advancing the camshaft by 4degrees you have to recheck valve to piston clearense , advancing the cam will lesson the running clearense between the intake valve and the piston , if you have enough clearense this will help your condition , not the best fix but usually helps. matching your parts correctly is the best way to go compression has to be increased as camshaft specs increase , hope this helps, this is a subject you could right a book about.

[@roaddog1973]

I have been round and round with my big cam low vacuum Cleveland too. Finally really started to get it figured out. If you are still having issues feel free to send me a PM and we can compare notes, share ideas.

[@marcstlaurent3719]

Thanks again Randy

[@holleycarbshowtosadvice40y90]

Thank you.

[@gorenas_garage]

Do you tune Holley carbs for rotary engines?

[@holleycarbshowtosadvice40y90]

I haven't but should work

[@NHRA1975]

Just wondering, if you are idling at like 1200 due to a lumpy cam (drag race application) would the power valve open because the system could be operating on the main circuit? I am only getting like 3 hg inches of vac...

[@jimdavidson5208]

yes

[@holleycarbshowtosadvice40y90]

The power does not effect the idle at all unless the power valve diaphragm is leaking.

[@shainedickinson2107]

Did you make other videos as well? This was great! finally makes sense.

[@holleycarbshowtosadvice40y90]

Yes I'm going to make videos on other mechanical things building other carbs and performance engines

[@holleycarbshowtosadvice40y90]

I have a lot of holley videos and will be doing different videos soon. Thanks

[@bobbyjordan2943]

What about the power valve plug to close it what does it help or do

[@holleycarbshowtosadvice40y90]

Bobby I usually block the rear power valve , the reason I do it is mostly for racing , when you accelerate hard the fuel in the front bowl runs toward the Jets and the power valve , fuel in the rear bowl runs away from the jets, and the power valve , the power valve sets up higher than the rear Jets so it's easy to uncover the rear power valve under hard acceleration , sometimes we have to add jet extensions and a special rear float to clear the jet extensions or we will starve the engine for fuel creating a lean condition, I do this for drag racing and other mods for circle track racing ,ps if you block the rear power valve say you have size 76 jets in the rear go up 6 to 8 jet sizes , in other words change to 82s or 84s will be close. Hope this helps.

[@Nomad-376]

@@holleycarbshowtosadvice40y90 Will this method work for a car that is not being raced ?

[@Nomad-376]

I've also heard of a bigger power valve and smaller primary jets . Your thoughts please. TY

[@kevinbarnes4068]

Thought on no power valve for drag race only?

[@holleycarbshowtosadvice40y90]

I always believe it's best to run a power valve in the primaries to keep control of your fuel curve, I've tuned 3 circuit dominators without power valves that performed great, what it comes down to is personal preference , you are always going to be fat down low and good up top straight jetted. I've seen carburetors with 4 and 5 fuel circuits perform amazingly great. Hope this helps.

[@theweldingmachine]

I have a Holley 600 one of the bowl are not filling up

[@holleycarbshowtosadvice40y90]

Needle an seat are stuck or out of adjustment, some type of blockage , bowl vent plugged or possibly interference with float binding. Hope this helps.

[@chrissramek7035]

Good job buddy.

[@holleycarbshowtosadvice40y90]

Thank you

[@mikec5971]

Good video thanks man

[@holleycarbshowtosadvice40y90]

Thank you.

[@brandoncherry1651]

How much you charge rebuild a carb ?

[@holleycarbshowtosadvice40y90]

Right now I'm not doing any rebuilds , maybe in the future. Have a great day.

[@lonecedarfarm]

if the PV is not working correctly,could it allow enough fuel thru to foul plugs and make engine run on 6 or 7cyl?Engine runs fine at idle then boat wont get up on plane(loss of power,plugs fouled.) 302 mercruiser engine 4 barrel holly.

[@holleycarbshowtosadvice40y90]

if the power valve diaphram is ruptured this will cause it to pull fuel out of the bowl , making the mixture extremely rich and foul your plugs, there is other things that make a carb run rich , to much fuel pressure , air bleeds stopped up, bad float , too big jets , don't forget about ignition , weak spark can cause an engine not to burn the fuel completely , these are a few things that can cause a rich mixture that will foul spark plugs. ps wrong spark plug heat range, hope this helps randy,

[@lonecedarfarm]

The boat ran fine earlier this year.I had a possible backfire in motor box,but engine never skipped a beat.When i idled down i noticed smoke or steam coming from the top of the carb.next day boat ran bad ,checked all plugs and all were wet and carbon fouled.mechanic has come all summer.has checked everything,firing order,new plugs, wires ,dist cap ,rotor,timing ,points.he has given up!!! carb was not checked thoroughly.Also gas or oil or both out exhaust, film on water.i am assuming unburned fuel.Have heard that a backfire can ruin a power valve.Is there a way to test the power valve?greatly appreciated.

[@lonecedarfarm]

also when you snap the throttle it will usually backfire or not be responsive at all.Trying to give you all the clues to the mystery.Thanks.

[@holleycarbshowtosadvice40y90]

if this happened just after a backfire through the carb , may have damaged the float and or the power valve, the brass float will not handle backfires , and will cause it to flood , or could possibly have damaged both pv and float, easy way to tell if floats are damadged is to compare with a new part . I like to use a nitrophil floats they seem to hold up better and are more foregiving, , ps be extra careful if carb is flooding that is very dangerous in a boat, always check and double check for any gas leaks, you cant be to careful, hope this helps

[@lonecedarfarm]

I ordered a couple of power valves(8.5) ,will try it this weekend. Both floats seem to work fine,the needle valves are fixed and cant be taken out, also no sight window screw,so not sure how you adjust the level in bowl. Did a test where you screw in idle screws and the boat continued to run, so i am assuming bad P.V . we will see.Thanks for the help.