vettepoor

Question about setting idle on dual carbs - I currently have the idle speed adjustment screw backed completely off on the front carb, and am using the rear carb for idle adjustment. Problem is, I cannot achieve a smooth idle. I adjusted mixture screws for maximum vacuum of about 19 inches, and cannot detect any vacuum leaks. Previously when I had an over-rich condition with the front carb, idle was better making me think it it now too lean. I'm just not sure how best to go about this. Any suggestions?

Thank you,

John

Plasticman

I do not have the same setup, so take this info with that in mind.

I have dual Carter AFBs, and use the front carb and rear carb idle circuits. That does 2 things:

1. It helps idle adjustment and smoothness (with a little more complexity).
2. Keeps some fuel flowing through the front carb.

The front carb idle speed adjustment is at zero (closed) and only the rear carb is used to set idle speed.

Good luck,
Plasticman

Frankie the Fink

I have run both ways - using both carbs' idle speed screws and using only the rear carb's. Didn't note much difference. I now run with only the rear carb's idle speed screw controlling idle and the front just barely contacting the stop...

First of all; it never hurts to "snug up" the carbs. Take a screwdriver and carefully make sure all the air horn screws are tight -- all 18 including the one under the vent cover cap. Not too much but snug. Same with the 1/2" carb mounting nuts; a short box end wrench on each one should help you not to over-tighten them...

Then with car at operating temp and curb idle speed...adjust PS rear carb mixture screw, then PS front carb mixture screw, followed by DS front carb screw and finally DS rear carb screw. Repeat -- keeping the car at curb idle as needed for highest vacuum. Then, do a slight "lean roll" by turning each mixture screw 1/16 to 1/8th turn clockwise...this may initially cause a slight roughness but it should smooth out after a drive. Make sure to keep the idle speed at spec so your centrigual advance weight don't start to engage and screw everything up.

You don't say, but if your car is the 270hp solid lifter with the Duntov high lift cam - its only ever going to be so smooth. It'll have a very mild 'lope' which is normal.

vettepoor

Thank you for the replies. I'm actually running a crate 350 with a fairly mild hydraulic cam, so I would think I should be able to obtain a very smooth idle. I initially set idle mixture screws at 1-1/2 turns open at all 4 locations. Went for a cruise, front plugs were showing very lean condition, while rear plugs were running slightly rich. I opened up front carb screws to 2 turns and leaned out rear carb in a similar manner. I know these carbs could be part of the problem due to age and wear, but I have been able in the past to get a smoother idle. Trial and error I guess! I will start over and try your method, Frankie. I read somewhere that these should all be initially set at 3/4 turn open, then adjust from there.

Frankie the Fink

My bad for not asking what motor you're using. Unless you are "in it" a lot the dual quad setup doesn't have the best fuel distribution in the world. Most driving is off the back carb's primaries...remember that when examining plugs. If you have the stock fuel lines then check that fuel filter carefully (and the air filter) for clogging and general junk (like rusty bits).. You might want to remoe those mixture screws and examine the tips carefully for dings or damage....they need to be pristine. When you do, squirt a small shot of carb cleaner in the mixture screw hole for giggles...