Quadrajet mixture screws
Thread Starter
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
Quadrajet mixture screws
I was adjusting the mixture on the Quadrajet today. 10 turns out to achieve the best vacuum at idle. That seems quiet excessive to me. Is that about the normal amount of turns you guys get? The car is an 81 with headers. The computer carb is gone and the motor is stock other than the exhaust. I plan on sending the carb off to Lars when it get some spare cash. The carb has a 79 ID number. I'm not running the computer either.
Race Director
Joined: May 2006
Posts: 18,708
Likes: 2,574
Sounds like way too much. Have you tried turning one screw 1/8 turn clockwise, check if the idle speed drops, then if not turn the other screw 1/8 turn. Continue this till you get a drop and then turn the screws back 1/4 turn.
Thread Starter
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
The carb was set at 11 turns out when I started adjusting it. I took the screws all the way in and started at 5 turns out. I tried starting it and it wouldn't run. I turned each screw out in half turn increments until it started and idled. That didn't happen until I got to 8 and a half turns. The vacuum was at 14. It's at 18 with the mixture screws out 10 turns.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
One of the mods I made on my q-jet when I rebuilt it per Cliff Ruggles book was to drill the idle screw holes larger. This makes the idle screws more sensitive to adjustment. Mine are now 3 1/2 turns out for best idle. You may want to try it. you'll have to take the carb off to do it or you may get metal in the intake.
Safety Car
Joined: Oct 2000
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From: Southern Indiana
Drifting
Joined: Mar 2011
Posts: 1,886
Likes: 9
One of the mods I made on my q-jet when I rebuilt it per Cliff Ruggles book was to drill the idle screw holes larger. This makes the idle screws more sensitive to adjustment. Mine are now 3 1/2 turns out for best idle. You may want to try it. you'll have to take the carb off to do it or you may get metal in the intake.
Don't ask me how I know. I hate to talk about it.
Last edited by gcusmano74; Nov 17, 2012 at 06:54 AM.
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Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
Yes, but only just SLIGHTLY larger. Read Cliff's book first, or send it to Lars. If you make the holes too large, you will lose the fine flow control that you need. You will have to find another throttle plate and start over.
Don't ask me how I know. I hate to talk about it.
Don't ask me how I know. I hate to talk about it.
Thread Starter
Le Mans Master
Joined: Aug 2010
Posts: 5,339
Likes: 546
From: Central Texas
I sprayed carb cleaner around the base gasket and the attached vacuum hoses. No change in vacuum or rpms. I've got some other issues with this carburetor. Hard starting, long warm up time (choke issue), and a bog in the transition from idle to part throttle. So I'm packing it up and sending it to Lars. 
Drifting
Joined: Feb 2005
Posts: 1,879
Likes: 38
From: Los Gatos CA
Vacuum Leak Test - Lars taught me a simple test. At idle, back out idle speed screw all the way (to close throttle blade). If idle speed does does not idle down and die, you have a vacuum leak. Air must be getting in somewhere.
Race Director
Joined: Jan 2000
Posts: 12,399
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From: San Jose CA
Also, to the OP - do you have a non-computer controlled distributor installed as well?