4 Barrel Quadrajet Air/Fuel Adjustment

[mntoy]

I reciently dipped my quadrajets carb, had the bushings replaced and rebuilt the carb. I am unable to adjust the idle down low enough. I can adjust it at cold start but when I drive the truck a 1973 C20 with a 350 and th400 for any lenght of time the idle seems to speed up alot. My question is do the two air/fuel screws (front drivers and passanger side screws) affect the idle too? if so is one air and one fuel or are the both air/fuel for each of the primaries? secondly which way is richer (more fuel) and which way is leaner (more air) screw in or screw out? Any help would be greatly appriecated. Thanks in advance.

[chevyjr73]

the air fuel screws can affect the idle but more than likely if the air fuel ratio is not correct it will idle too low not too fast. sounds like you may have a vacuum leak somewhere or you have something sticking. turning the screws in leans out the mixture and turning them out richens the mixture. i always back mine out until i just do get the highest amount of rpm's and then give them about another quarter to half a turn.

[mntoy]

How can I find the vacuum leak?

[dswancutt]

I would first check all the vacuum lines to see if they are properly hooked up and the hoses are in good shape. I have heard of spraying carb cleaner at the base of the carb to check for leaks between the carb and the manifold, but someone else can confirm or deny this. Did you replace the gasket between the carb & manifold?

[chevyjr73]

spray some carb cleaner or wd-40 around the carb. if the idle pick up you have found your leak. also have you checked to be sure that the choke is operating properly?

[mntoy]

I have not been able to locate a vacuum leak, I sprayed carb cleaner around the carb and manifold but have not heard an increase in engine rpm. I also do not see anything sticking and the truck seems to run good the rest of the time just a fast idle. Could I just be missing the vacuum leak? Any additional help would be great.

[85 K5]

Another check for vacuum - can you kill the engine by placing a rag over the top of the carb? If not there is a leak.
When the choke is operating it engages a fast idle cam. Is this connected and operating properly?
Do the throttle plates close completely?
Is the idle speed adjustment screw backed out all the way? If so there is too much air coming from somewhere.......Hope some of these thoughts help.

[Duro5341]

Originally Posted by 85 K5

Another check for vacuum - can you kill the engine by placing a rag over the top of the carb? If not there is a leak.
When the choke is operating it engages a fast idle cam. Is this connected and operating properly?
Do the throttle plates close completely?
Is the idle speed adjustment screw backed out all the way? If so there is too much air coming from somewhere.......Hope some of these thoughts help.

Alot of good suggestions here^^^^^

[mntoy]

Thanks for all your suggestions, from previous messages the carb was dipped and the bushings were replaced, I have all ready checked for leaks with carb cleaner with no success, checked all hoses all are new and a new carb to manifold gasket, the engine will die real fast if I place a rag on top of the primary butterfly, the idle screw is all the way out and it appears if I look down inside the carb the primary plates are all the ways closed(best I can see with a flash light), the choke is missing one rod from the thermostat to the side of the carb but the top butter fly is completely open, not really sure how to adjust the fast idle cam but the screw for it on the passanger side of the carb appears to be most of the way out and I will purchase the choke rod this wednesday when the carb shop opens. I adjusted the idle screw out all the way and then started backing out the fast idle screw and then went back to the idle screw and backed it out some more trying to get the idle as slow as possible, again the primary plates appear to be completely closed. To me it sounds like a vacuum leak but I cannot find it. Again the truck runs fine and at cold start idles ok but after a 15 minute drive the idle is to fast and I cannot adjust it down any less. I am just about ready to take it to a shop and pay someone to look at the carb or put a Edelbrock carb on. Any ideas what I am doing wrong or how to go forward? Or any suggestions on a shop in Southern California? Thanks again for all your help.

[jimfulco]

I'm leaning toward a problem with the choke linkage. If you can't get it straight, there are a number of carb shops in SoCal. The Carburetor Shop is in Ontario, and Sean Murphy Induction & Jet Performance are both in Huntington Beach.