Seff

My idle screws are close to the point of coming out, and the car still idles, starts and runs great. Am I overlooking something, or is it time for yet another carb rebuild?

'72 350 4bbl, '71 Q-jet on it, new gaskets.

droptopron

Are you sure you are turning the idle screw & not the mixture screws? The idle screw is in front of the linkage and is mechanical in nature. You turn the screw the idle changes.

http://www.hotrodders.com/forum/quad...ion-63050.html

The mixture screws are at the bottom of the carb to the left & right of the fuel inlet.

Oldsmaniac

Turning out the idle mixture screws in on the front of the carb richens the mixture. These are used for lean/richening the idle circuit and should not be used for idle speed which is controlled by the screw near the linkage connection at the carb. Why are the screws out so far?? Usually they should be turned out about 3-4 turns.

oddball

Hey, if it runs well....
I just ran through several carbs last weekend, and one main body didn't idle well, while another one did. Same modifications and internals to both. Strange, but we're talking 40+ year old cores here.
That could also mean there's a minor air leak somewhere, so the idle screws have to be really rich to compensate.

Seff

Because I thought that turning them IN would richen the mix! No wonder my car smells a lot!

The idle being affected was something I read, about adjusting the screw until the idle gets rough, then turn it a turn back.

Oldsmaniac

I just re-read my post and let me be clearer. Screws turned in - less fuel or lean. Screws out richer mixture.

Seff

I got you, I was just elaborating upon my flawed thinking, as I thought it was the other way around. :P

Pretty sure you just solved my problem.

MDchanic

The idle mixture screws restrict the flow of fuel through the idle ports as they are tightened.

There are other carburetors, including a number of European ones, that have idle air screws, which restrict the entry of air as they are tightened - I suspect that that is the reason for the confusion.

- Eric

Seff

Could be. In any case, I'll tighten the screws instead. A good rule of thumb is to tighten until the idle changes, and then a quarter turn out, yes?

Lady72nRob71

I think the Holley 5200 used on the Ford 2.3 is like that, or at least that is how it acts! I have never got a real confirmation on it yet...

4rims

My screws were out too far on my Rochester and it was causing a stumble when moving from a stop to taking off. I turned both in all the way and backed out 2 1/4 turns, set the idle screw and is running nicely