54greg

Just got my Quadrajet rebuild. What's the procedure to adjust the air fuel ratio?

plaidside

Are you talking about the idle air/fuel mixture, cruise A/F or wide open throttle A/F?

63split63

If you had it rebuilt by a professional the 2 idol mixture screws are probably set at 1 1/4 to 1 1/2 turns out . Warm up the engine and set the idol speed where you need it , hook up a vaccume gauge to a full time port , slowly adjust the mixture screws 1 at a time untill you get the best vac reading .
You will probably have to keep adjusting the speed screw as your idol speed will change as you adjust the mixture screws .

Bill

DansYellow66

Between lean roll off and rich roll off there will be an area over which you won't see much if any variance - maybe a half screw turn give or take some. Unless you have a really wild cam in the motor it's best to leave it close to the lean roll off point. It will reduce the chances of popping through the exhaust during deceleration and maybe run a bit cleaner at idle.

jim lockwood

Not sure I understand this and it doesn't match my experiences.

I cured decel popping by going to a richer idle mixture on my GS vintage racer. Cam makes good power but isn't wild by anyone's definition.

Jim

tbarb

Dan,

I gotta agree with Jim on this one.

My thinking is that the pop is caused by a high vacuum condition during deceleration. This high vacuum pulls lean fuel from the transfer slot/idle circuit and also scavenges from the exhaust further leaning the slow burning mixture. Some of it is still burning when it hits the hot exhaust and pop.

Frankie the Fink

Precisely - fought this on my '67 Chevelle SS-396 for a month or two.
Drove me nuts...

plaidside

The question still has not been answered as to whether he means idle mixture or A/F ratio for power or cruise.
Joe

cardo0

Carbs are fairly rough in metering a/f mixture needs. The idle mixture screws adj only the ....... idle mixture of course and if your looking for cruise and WOT then u need to select the correct jets and rods. To bad u didnt rebuild your own carb then u would understand jetting and mixture rods. BTW those mixture screws are not user friendly and i find it best to adj them both the same amount of turns at the same time - they've had me chasing my tail (most of) many times.

Eyeball tuning leaves u with changing jets to make it rich enough to blow black smoke out the tail pipe (yes u need a backup driver behind u) then reduce jetting just 1 or 2 steps/numbers. Choose the rods for a smooth transistion to full rpm/power.
Pump shot makes a stumble when drunk with fuel or a bog when too lean.

Ok using real science with a a/f monitor the wide bands will get u into the 12ish a/f ratios for best power while the narrow bands leave u at 13:1 a/f. But the w/b's are twice the price. And the limits with carbs is that they have no density compensation meaning that if u tune in hot summer u may find yourself lean in the cold dense air of winter (using w/b or n/b O2 sensor). BTW heated HEGO is the best (say $400).

Thats enough blab for u for now. Let us know when u need more help.
cardo0

54greg

I can take apart and rebuild most anything, I think most on this forum can, but for some reason I have a mental block on carbs. My rebuilder said everything was adjusted BUT I would have to set the air/fuel mixture. So I really dont know the difference between the mixture, cruise A/F or wide open throttle A/F.

Looks like I have some reading to do.

Thank you for everyone's guidance

cardo0

FYI Cliff Ruggles has the best book on Quadrajet carbs.

Hope this helps ya greg.

54greg

Thank you

troutster71

Goin' out on a limb here 54greg, but I think your rebuilder means to adjust the idle mixture screws, in front, in the baseplate. Reference post #3 from 63split63. Std carb oh procedure.

Engine fully warmed up, vac gage or rpm meter hooked up.

Before starting engine, verify the position of the IM screws by counting the turns in, to LIGHTLY bottom the screws out. Return them both to original psn or to 1-1/4 to 1-1/2 turns out, but both the same. Mark a small screwdriver and go in 1/4 turn increments, (it helps me).

Start warm engine, set idle speed. Go in 1/4 turn increments screwing IM screws in, (cw) until engine starts to lose rpm or vac. Then back screw out for best/highest idle/vac. Do this to each screw.
Reset idle speed.

Test drive. Slight hesitation or small stumble MAY be helped by going slightly richer ,(out) on IM screws. Your conditions may vary.

This is basic, but a place to start.

I'm sure someone will post and tell me by limb is about to break!

Tell us how you do, GA

54greg

I think your right. Thank you

DansYellow66

Mea culpa - you are right. I guess fatigue caused me to confuse popping and exhaust backfiring symptoms in my befuddled mind. I still like to set the idle/fuel idle mixture t a little to the lean side - short of the idle rpm falling off any or creating any drivability issues.