Finding the right Jet Size and Accelerator Nozzle Size - Holley Carb

[@JL-og9gm]

You may already know this, but the main jets in the primary are supposed to be sized for cruise at constant rpm. The best way to do this is to NOT pick an AFR that you want to run at. The best way is to let the engine tell you what is best. The way to do this is to run a vacuum hose from a FULL vacuum source on your carb or manifold, to inside your car so you can read it while you are driving. The object is to find the primary jets that give you maximum vacuum reading at a constant speed of around 60 mph. Note this maximum vacuum reading. That's your ideal primary jet. The lean spot you are experiencing is likely being caused by the power valve. The rating on your power valve is too low. The job of the power valve is to enrich the mixture when you press on the gas from a steady speed. This is exactly the area you need more fuel. The power valve should be rated to a minimum of 2 in-hg below your constant speed vacuum you noted before. If your vacuum at cruise is 12 in-hg, then 12-2=10, so you would need a power valve rated as close to 10 as possible without going over 10. That would be a power valve rated at 9.5 in-hg. I hope that helps. By the way, based on your build, your carburetor is absolutely NOT over sized. It just needs tuned.

[@AndyKruseChannel]

Cool, thanks for the heads up, I'll have to look into it. :)

[@hymugglee6236]

Bad info, a power valve does not cause lean spots in and of itself hence the name power valve. Idle circuit to accelerator pump then to main jet then to power valve 'under a load between 1/3 and half throttle and then to secondary side if equipped. Accelerator pump is the transition that causes dead spots

[@SlowInFastOut1776]

Glad you're documenting your carb adventures - that's some black magic that I'll need to learn once mine is up & running. Looks like you're getting closer.

[@AndyKruseChannel]

Yeah, it is black magic, and I'm constantly going over everything because I think I've got it figured out, then I don't :)

[@pauls7751]

Hi Andy another quality video. I have a built 347 in my 65 fastback. Originally I had a 750 double pumper and the engine was never happy and idled so rich it burned your eyes. Finally after a couple years I dropped down to a 670 street avenger vacuum secondaries and it was like magic. The engine loved it however I've continue to tweak it. It came with .065 main jets but my plugs showed a rich condition, I have no AFR gauge. Anyway I dropped jet size to .063 and it didn't like it so I put the 65s back in. I may even try .067s later. My squirter size is .031. Anyway it's back to how it came. I have no lean bog and it runs great. I think the plugs looked rich because too much idling in the garage during periodic start ups. AFR gauge is next on my list. I use to have a lean bog but solved it tweaking the transition circuit with more air.

[@AndyKruseChannel]

Yep, there's lots of options out there to dial in the cars. :)

[@pauls7751]

@@AndyKruseChannel I have a heck of a time trying to adjust the idle mixture screws using a vacuum gauge because the needle jumps around a lot. Does your new motor do the same thing? A mechanic friend told me it was because of the big camshaft. I guess the overlap or duration.

[@AndyKruseChannel]

@@pauls7751 Yes, I have the same issue, but it's because of the cam. The overlap on my cam, at idle, causes the vacuum readings to be difficult to decipher. :)

[@chevroletbelair1]

@@AndyKruseChannel i have build a 331 with a retro fit hydraulic roller comp cams advertised 290/290 with 230/230 on 50 (110) and even i can adjust idle speed to be flat on 900 rpm without fluctuation,. you just need to put idle mixture screws on highest vacuum reading and from this you can set the idle mixture screws more in to get 14.5 afr. i learned on thunderhead 289 youtube channel that idle is +-14.5 cruising +-14.7 half throtlle is +-13-13.5 full throtlle is +-12-12.5 afr

[@johnc7431]

Andy gorgeous car. I stumbled across your channel I like how you talk through step-by-step. And explain everything. I am actually going through the same steps with my 302. I will continue to watch your progress like I said before your car is gorgeous

[@AndyKruseChannel]

Welcome aboard and much appreciated!

[@donsmanufactory]

Hey Andy, If you can find a 2 stage power valve it will allow you to run leaner and not experience lean / flat spots. My 85 5.0GT used a 2 stage to solve that problem. I implemented the same tactic in my 66 coupe 302 w/ 650 DP holley. Better economy, the plugs stay clean and smooth throttle response.

[@AndyKruseChannel]

I may have to look into that. :)

[@macfranks2456]

The 750 is probably just about right for your engine it may be a little fat at Cruise but at wide open it'll probably use every bit of that on that engine.. you can't do everything in one video but it's always good 2 try the accelerator pump cams first because you can pick one that is more aggressive on the initial part of the shot if it's going a little bit lean.. you can find those colored codes to Fuel Curve plots in a little diagram online it's a holley diagram.. I guess I'm a bit late to be commenting on this video sorry LOL

[@AndyKruseChannel]

The 750 is really only helping me at the top of my RPM range where I don't spend much time. I might do more of that high RPM stuff down the road, but for now it's almost too big. The math says I need a 682cfm Carb at redline, so a 700cfm unit would be a better fit. I have yet to tweak the accelerator cam, but it's on my list of things to do. I'd like better throttle response and the accelerator pump and cam are things I'd like to explore. :)

[@wolfthornhawkridge5705]

You nailed the exhaust note! Car sounds great.

[@AndyKruseChannel]

Haha, thanks!!

[@jjones7831]

I bought a QF Slayer 600 and the stock squirter was a .018. I bought a .025 and a .028 with a cam kit and see what happens. I was getting the bog big time off idle, love your car, man...

[@AndyKruseChannel]

This just shows that a specific engine with a specific carb and specific injector nozzles and cam profile may not be the best choice. Some people have figured out what works for specific engine builds, and then there's everything else in between. While my engine guy told me what carb goes best with the engine he built, he didn't tell me how to dial it in. This is the stage I'm going through right now, trying to find out what works, but more importantly learn from my mistakes. :)

[@Bige4u]

Tuning carburetors can be tricky, finely tuned one day, almost a nightmare the next no thanks to ever changing weather conditions, but using a vacuum gauge and wideband 02 sensor is a must. Ever think about using a double pumper instead, from what i've read, its just about 100% recomended to be used with manual transmissions cars, beter control of the secondaries when you need them, versus vacuum secondaries trying to figure what color spring is needed.

[@AndyKruseChannel]

I've heard the same, but I'm going with what my engine builder recommended. :)

[@christophersambuco9414]

This carb tweaking adventure is going to make for a great video series. Maybe some power valve tweaks too? Anyway, thanks for sharing 👍

[@AndyKruseChannel]

I'm not sure I need to adjust the power valve yet, my vacuum reading matches the power valve number in the carb already. :)

[@chevroletbelair1]

@@AndyKruseChannel just try a bigger number powervalve if you have it in you're box. Always can go back

[@ginols68]

great content andy keep it up

[@AndyKruseChannel]

Thanks, will do!

[@LegacyGarage]

You really want to feel a difference play with front pump cams, try a green and it'll wake up the hit

[@AndyKruseChannel]

Thanks for the heads up. :)

[@shoujiki999]

You didn't install the washer on top of the pump nozzle. You can see the old one sitting precariously at 10:50. Hope you found it and fixed that up before installing it all again!

[@AndyKruseChannel]

Probably, I don't know, this video was made quite a while ago.

[@devinl7543]

While the carb size may or may not be ideal... I'm no expert... being bigger actually makes it run leaner, not richer, so that shouldn't be why it's running richer than you want on cruise. For the pump shot, you can also change the pump cam as well as which screw hole you use, and you've still got more nozzles you could try too.

[@AndyKruseChannel]

The carb is perfect at WOT, it's right where I want it. But at partial throttle it's too much, that's where the fine tuning of jet size (as well as the accelerator nozzle and the power valve) comes into play. :)

[@coyoteserranomusic]

Are both jets supposed to work at the same time? For a 2 bbl? Or is one a idle jet and the other a main jet.

[@AndyKruseChannel]

One jet per barrel, but they're adjusted independently despite both be used at the same time.

[@coyoteserranomusic]

@AndyKruseChannel So like if I press on the Accelerator, I'll see one stream of gas In each barrel? Sorry stupid question.

[@AndyKruseChannel]

@@coyoteserranomusic Correct, if you look down the Carb and blip the throttle, you'll see fuel spraying in each barrel. :)

[@coyoteserranomusic]

@AndyKruseChannel 🤣 kinda thought so. Was running my 70s dodge with no fuel filter.. must of got junked up.. thanks man! I don't have much experience with carbs.

[@orezamora8018]

so, did you ever go back to adjusting the accel pump nozzle? In your video it looked like it snapped into 16, but back to good. Im dealing with the same thing and am thinking of going to 37 on my nozzles based on your video. I have a lean condition that seems to happen longer than a second.

[@AndyKruseChannel]

I have not. I didn't get to drive the car much between that video and now, and I have been waiting for my car to get done at Paint, so it'll be a little while longer. I am hoping to get the car dialed in this spring. :)

[@chevroletbelair1]

which powervalves do you have in the car when shooting this videio ? and which sizes do you have in the box extra?drive on cruisespeed and put a litlle more throtlle in so to get 5mph more speed and see with a vacuum gauge wat it reads . if the stumble lean spot is around 10 vacuum you need a 9.5 or 10 inch powervalve . this solved my exact tesame problem. when cruising you have vacuum but when you put a litlle more throtlle in vacuum almost is gone and so the jets cant pull fuel enough and so the powervalve has to come open so the extra fuel can go thru the pvcr's (located behind powervalve) and give the engine more fuel at this low vacuum signal

[@AndyKruseChannel]

I don’t recall, that was quite a while ago. But thanks for the heads up. 🙂

[@1Coolbanana]

Go up one size in the power valve and 2 sizes smaller in the main jets.

[@AndyKruseChannel]

Thanks for the heads up. :)

[@1Coolbanana]

@@AndyKruseChannel Did you ever get any further with the tune. Cant seem to find any of your videos discussing it anymore?

[@AndyKruseChannel]

@@1Coolbanana Not done yet, hoping to get back to it this spring/summer. :)

[@gunnernepo]

Hello, I noticed your black stainless braided fuel line. What brand is it and where did you get it? I’ve seen similar ones on Amazon and eBay but with mixed reviews about leaks, fuel pressure gauges not accurate and the center hose too long. Appreciate it, thanks!

[@AndyKruseChannel]

@@gunnernepo If you're talking about the fuel line kit that connects to the two port on the Carb, it's from Summit Racing, part number SUM-220101B. It comes with a T-fitting to attach a fuel pressure gauge, but I don't recommend using it (it's weak and you can install the gauge too far and break the T), I just added an AN piece before the T to accept a pressure gauge. If you're asking about the fuel line from the Pump to the Dual Feed Line (the kit I described above), I designed and sell that fuel line kit. It has the required AN fitting to install in the fuel pump and the AN fitting to attach to the dual feed Carb kit. You can find it here: krusebuilt.com/products/fuel-line-kit-silver

[@nickmcminn2137]

Did you try a different pump cam first? Sounds like you need a higher number powervalve. What is your AFR at WOT? 1 change at a time

[@AndyKruseChannel]

I'm not focusing on my AFRs at WOT, I'm trying to lean it out a pinch on partial throttle. :)

[@nickmcminn2137]

I was just curious because lean WOT is it really hurts your engine. If you worry too much about leaning it out at cruise it will starve where it matters. Just a thought 👍

[@GeoffTheGiraffe]

@@AndyKruseChannel Hey Andy. I was having the same issues with my holley and I ended up needing to adjust the Idle feed restrictors to lean out the part throttle transition circuit before the main jets come on. Now I cruise around part throttle in the mid to high 15 AFR and get great gas milage

[@SmackeysGarage]

You need EFI 😁 It's been something I've been debating trying out.

[@AndyKruseChannel]

Yeah, it'd be nice on one hand, but I like tinkering with it too. :)

[@rogerdeneault6560]

Andy you need to change the power valve.

[@AndyKruseChannel]

Not sure about that yet, the vacuum reading points to the power valve number that's in there. :)

[@Lunar_Stoner]

​@@AndyKruseChannelTheir is allot more to it I think there's power valve jetting to dial it in vs just a power valve I think holly has that awesome jetting too.

[@LENSO_CHEVY_LT1]

That 1 second 16ish to 1 condition youre seeing isnt going lean, its actually rich. I made this mistake for ages and learnt it from another youtuber. Its actually unburnt air from a rich condition giving a false lean condition and thats why it goes rich after. Try a smaller acca nozzle

[@AndyKruseChannel]

Yep, working on it.

[@matthewhummel6023]

Learn about how the nozzle size affects the power band going from the transition circuit to the main circuit in your beast.

[@AndyKruseChannel]

Ok

[@stevenbongiorno9277]

Have you ever heard about raising the bowl level slightly, so your transition happens a little faster

[@AndyKruseChannel]

How does the fuel level in the bowl aid in the transition from idle to throttle?

[@stevenbongiorno9277]

@@AndyKruseChannel the main system is fed by vacuum pulling fuel from the bottom of the bowl. So, if the level is low, it’ll take a little longer to pull the fuel. If the level is a little higher, it will respond a little quicker. The main system siphons from the jets at the bottom of the bowl

[@gmstudent93]

14:1 is a good fuel economy number and 12 -12.5 under load

[@AndyKruseChannel]

Yeah, still chasing the good fuel economy number, I'll get there some day. :)

[@darringrey4329]

No incorrect that is correct fuel air ratio , not the correct lean Cruse I can run at 16s

[@trinogt]

I’ve researched that you can safely run 15:1 afr at cruise since there is very little engine load, even a bit higher… it’s at load and WOT that you want to get no more than 13:1 air. I’ve got my 351 with 600 Brawler right at 15:1 cruise speed, now working on slight stumble still, after going up to 35 pump nozzle. Might try one more size bigger.

[@rogersome1413]

What is your combonation?

[@AndyKruseChannel]

Combination of what?

[@tonycolca2241]

Holley carbs are great for racing applications ifvyouvuse a dyno you have a lot ofvwayscto fine tune a holley the higher priced models have changable air bleeds power valves sone even have changable emulsion tube jets endless tuning adjustments. A hetter carb for the street us edelbrock avsc2 much easierti dial in (most of the time good roght out ofbthe box) keepa adjustments gives in most cases better fuel milage. I didnt catch what size engine you have most tend to put too big ofca carh on. Cu in x rpm x vloumetric efficiency sock is about .85 modified engine .90 highly modified 100%then divide by 3456

[@AndyKruseChannel]

ok

[@rogersome1413]

Step up in power valve size like from a 6.5 to an 8.5.

[@AndyKruseChannel]

Nah, that's not what I need to adjust for the issues I was experiencing.

[@jasongriffin5994]

68 main jets will make it perfect

[@AndyKruseChannel]

Perfect is subjective. :)

[@tonyscherer8281]

Why not contact thunderhead 289 and try his carb cheat

[@AndyKruseChannel]

Because his App doesn't change the Jet size, the Accelerator Nozzle size, or the Power Valve size. All three of these items are used to tune the carb and change the rich and lean conditions that are created with the a specific carb size and the fuel needed to run properly. :)

[@babygrand1100]

Next time take the butterfly plate off

[@AndyKruseChannel]

The choke plate? Nah, this worked fine. :)

[@babygrand1100]

you know it snaps off/on right?@@AndyKruseChannel

[@craphittingthefan2360]

No such thing as too much carb if you know how to adjust it. 🤣

[@AndyKruseChannel]

Understood. 🙂

[@darringrey4329]

Mate you are so far off the mark, 13s at cruse, 2000,2500 rpm , the engine dosnt make enough power to do any harm at that speed, i run at 16s and or a slight hesitation which comes and goes , i get 20+mpg on 620hp 1050dominator street car your mechanic must be 105 tunning has moved on since his model T days

[@AndyKruseChannel]

Thanks for the info. :)

[@Gumshoe1911]

Why does everyone have to play horrible music for good videos???

[@AndyKruseChannel]

Not sure why your choice of music is the only good music out there.....