Carburetor Idle Mixture Tuning

Carburetor Idle Mixture Tuning. In Part 2 of my series I show you how to properly tune your idle mixture circuit on your carburetor using nothing more than a tachometer and vacuum gauge. I also show you the proper fuel pressure and how to adjust your fuel pressure should it be too high or too low. All of this is done using an Edelbrock AVS2 650cfm carburetor but the same theory applies to Holley, Quick Fuel, Brawler, Demon, etc.
WATCH PART 1 FIRST. If you haven't seen the first installment on how to properly install an Edlebrock carburetor check the link below: • How to Install an Edel...

[@splooshes2]

you helped me understand a decent amount in this bro thank you. I still don't know shit but I'm learning so thanks!

[@SpeakerKevin]

I installed the pressure gauge in the spare outlet on the pressure regulator, works great.

[@leedan9887]

Like your car man

[@carlosmolinares4329]

Me encanta la mecánica

[@bluecollarfox916]

What’s your opinion on ported vs manifold Vacuum for the vacuum advance signal? With my n91 cam in my SBF I’m unable to get it to idle at a high enough rpm without opening the throttle blades too much and exposing the transfer slots. Guess I could open the secondaries but I also noticed I was able to get a leaner idle mixture with more advance at idle (manifold vacuum signal). Just what worked for me so far. Love the content.

[@truckandroll989]

So Ive run both. From my recent research it appears in most cases Ported is preferred except in a situation like yours where you need the extra timing for improved idle quality. The other option would be to modify your distributor to limit total advance so you can run more base timing. Lets say your average distributor adds 20 degrees and you typically run 14 initial… whelp thats 34 total. With a big cam you may want 18-20 initial but not 38-40 degrees total. There are ways to modify it so it stays at 34 total but it definitely involves more tinkering.

[@bluecollarfox916]

@@truckandroll989 awesome thanks. For its staying on manifold vacuum haha.

[@trailerparkcryptoking5213]

MANIFOLD VACUUM for those with a brain!!! You noticed it increased idle rpm which allowed you to lower base idle and cover up those transfer slots. Don’t drill your throttle plates, crack open your secondaries just enough....

[@360escapades]

Very good

[@DjTragik951]

First thing is to adjust air mixture screws then the RPMs? When I set the mixture screws to about 17 in m. My rpm is about 1400 then when I bring the rpm down so does my vacuum it goes to about 15” m. I’m still trying to learn how to adjust my AVS 2 carb

[@truckandroll989]

Out of the box the carb is going to have some baseline settings. Assuming your fuel pressure is good, engine is fully warm (choke open) id likely get rpm set first and then start adjusting IMS. Adjusting them will affect idle and also vacuum. You are shooting for highest vacuum at desired idle rpm.

[@indarbhamra4480]

Hi, what kind of vacuum gauge do you use, if possible please provide a picture. Thank you. Love your blog. Best regards from Kenya.

[@truckandroll989]

Any of them really work. Preferably one with a larger face so it is easier to read. Here is an example www.amazon.com/Hromee-Carburetor-Pressure-Diagnostics-Leakage/dp/B08P4P97H2/ref=mp_s_a_1_1_sspa?crid=2UVQX0FET4VKU&keywords=automotive+vacuum+gauge&qid=1661361259&sprefix=automotive+vacuum%2Caps%2C121&sr=8-1-spons&psc=1

[@garyweber7139]

Many classic cars specify the car is to be in drive if it's an auto trans when tuning, just remember to chock the wheels and don't rev it.

[@truckandroll989]

Yes, this video was specific to a manual so neutral is fine but yes for an auto. Or have someone hold their foot on the brakes

[@vernm6189]

I run my fuel press. at 6.5 without any issues, with the same 289/306 engine. @ 14" manifold VAC. 1st on Race Day !

[@armygreenfj3924]

Hey I just bought the edelbrock AVS2 650. Im in the process of rebuilding my 350 with the edelbrock e street top end kit for my 75 blazer. The fuel pump i ordered is a edelbrock mechanical fuel pump rated at 6psi do i still need a fuel pressure regulator?

[@truckandroll989]

It certainly doesnt hurt to add one. Edelbrocks are pretty susceptible to fuel fluctuations. You could try a fuel pressure gauge first to see what it reads and determine after that if you want to add a regulator

[@armygreenfj3924]

@@truckandroll989 thanks

[@josephmclaughlin9787]

Yes because sometimes the pumps flow more then Advertised

[@armygreenfj3924]

@@josephmclaughlin9787 or sometimes the gauge is reading wrong.

[@sergiogarcia3719]

Apresiate it

[@sergiogarcia3719]

I am runing 3.5 psi on my 1406 edelbrock and is not working properly is the presure low i need 5.5 to run beter

[@truckandroll989]

Yea I would turn it up and that should help. Between 5-7 is good

[@darrellsomers5427]

Way to low the more the better never seen one as low as yours

[@jm-ve8ry]

What about briefly hooking vacuum gauge to ported vacuum just to make sure that at idle, you have no vacuum. If idle speed screw is turned in too far you'll start to get vacuum at the ported spot.

[@truckandroll989]

You could do that if you wanted

[@corydillon9864]

What was the brand or part number for the fuel guage

[@truckandroll989]

Fragola performance is the brand. Got it off of Summit Racing. Part #: FRA-900001

[@CruisinComPTon]

Is it better to do this with an air cleaner on or off?

[@truckandroll989]

Ideally on but thats usually hard to do just for access sake

[@omarespinoza1804]

No issues without a fuel return?.

[@truckandroll989]

Nope none at all. I believe you can run this either way

[@omarespinoza1804]

@@truckandroll989 good to know.

[@Mr1974dust]

Is it really 100 degrees in your shop if so im sorry .

[@truckandroll989]

Lol it very well could have been! Tall ceilings, no fan, and Utah summer heat can be no joke… despite us being at 4500ft elevation!

[@Mr1974dust]

@truckandroll6466 Where do you live Im in sanpete County ut mt pleasant

[@truckandroll989]

@@Mr1974dust i am in SLC

[@darrellsomers5427]

Never seen a raal cam make 25 inches if vacuum lol

[@davidkeeton6716]

Stone stock, grandma ride cam. Go up to 285-294 duration range you are probably going to see 10-12 at best.

[@JetBomb-xx6ye]

Best thing usually to do is to take it to the professional

[@truckandroll989]

Lol but why? With the initial investment of a couple tools theres no major secret. Between this video and a few of my others I show you how to get your carb setup properly without the need of a “professional”. Moreover, nowadays you are more likely to find someone who can tune your EFI but is a bit rusty when it comes to a carb. Following this same logic I tuned my 65 C10 before taking it to Westech Performance (Engine Masters, Roadkill, Steve Brule, etc.). They told me my AFR (carb tuning) was perfect and just addressed timing when dyno tuning. It isnt rocket science.

[@gypsy6211]

Professional? It's a carburator. You walk into an autoparts store and ask for a long fuel filter for a quadrajet, they'll give you what looks like an oil filter for an ATV. Professional carb tubers? There's very, very few left in the country, they've all retired or passed. Today's 'Professional' isn't much more than a computer reading, parts cannon loading, tech.

[@JetBomb-xx6ye]

@@gypsy6211 yeah im sure you know by the sound of it 👌

[@bottmar1]

I don't agree at all with turning the mixture screws in a fraction from highest idle. That is how all the carbs. were set at the factory in the 70's. There was a hell of a lot of rough idling engines because a worthless lean setting back then. When left this way you will end up re-adjusting them more often because even a little contamination in the idle circuit will create a leaner condition causing a bad idle. A slightly lean condition will cause a rougher idle than a slightly rich condition. Turning the mixture screws out a quarter turn rich gives you a fudge factor against a lean condition so you won't see your customer back in your shop as often. I have adjusted mixture screws a little rich for 53 years and don't have see my customers as often. They like that better. (And no, to you guys that think you will gas foul the plugs. You are wrong)

[@truckandroll989]

I dont think thats bad advice. One could argue you can still shoot for highest vacuum but perhaps not lean them out past that point?

[@bottmar1]

That's better then setting them the government E.P.A. method but barely. I new what the E.P.A. guide lines were by noting the mixture screws position when adjusting the new cars back the seventies after I pryed those damn plastic caps off and found them about a sixteenth or slightly more turned in from highest idle speed. I put the kibosh on that and turned them out from then on 1/4 turn. I'm glad you seem to understand the value in my method. So many KZbin warriors take the "book" method to heart but you are more flexible. Thanks.

[@truckandroll989]

@@bottmar1 and so i better understand the issue back then (and likely now) was the small orifice created by the lean IMS setting was prone to being further reduced by debris which could cause the need for further adjustments/lean stumbles off idle, etc. I doubt a 1/4 turn made a very big difference in AFR/idle speed/vacuum but it sounds like it reduced the need for more regular maintenance.

[@bottmar1]

That is correct. You get a little more time between adjustments without runabilty problems. Catch cha later.