Idle mixture
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Instructor
Joined: Nov 2019
Posts: 245
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Idle mixture
Pulled the plugs (AC43’s) on my 68 327/350 hp and some were black. After a full tune up (AC45S) the engine runs great but I think was running rich.
The carb was replaced back in 1970 by a previous owner due to a leak. Current carb is for the 350/350 hp, just maybe the carb needs some mixture adjustment for the 327 motor?
The mixture screw is on the lower front of the carb? What is the adjustment?
The carb was replaced back in 1970 by a previous owner due to a leak. Current carb is for the 350/350 hp, just maybe the carb needs some mixture adjustment for the 327 motor?
The mixture screw is on the lower front of the carb? What is the adjustment?
Pro
Joined: Nov 2019
Posts: 646
Likes: 351
From: Portland, Oregon
Mostly they like it just a little rich, but not always. Using a tach or vacuum gauge set idle screws to get max rpm or vacuum, then out just a little for rich, in just a little for lean. To test for rich pull a little vacuum hose loose, if the idle goes up it’s running rich. If it’s inconsistent or been a few years might consider a carb rebuild. Call Cliff’s High Performance for kit or parts. Check all the old carburetor posts here, especially the ones Lars comments on.
Tech Contributor
Joined: Aug 1999
Posts: 14,356
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From: At my Bar drinking and wrenching in Lafayette Colorado
Thanks for the kind words, Charlie., 
In response to the OP's question about, "What is the adjustment?" - There is no one, set adjustment on the mixture screws. A Q-Jet, depending on year, carb number, and setup can have a "correct" idle mixture screw setting anywhere from 3/4 to 9 turns out on the screws from their seated position. Even 2 carbs with the same carb number will have different setting requirements. Without use of an air/fuel gauge, the easiest and quickest setup method is as outlined in your GM Service Manual, known as the "idle lean drop" method. With both screws set to the same setting, adjust them evenly to obtain best quality idle. Once achieved, screw both in the same amount to obtain a 50 rpm drop in idle speed. This is the factory recommended setting for best emissions. For best throttle response and operation, as Charlie noted, a slightly richer position is desirable. So, from the 50 rpm lean-drop position, simply back both screws out about 1/4 turn to allow the rpm to return to best idle position. This, of course, assumes that your carb is otherwise correctly set up and in good operating condition to allow the idle system to function correctly. Most carbs, if running poorly, have multiple problems and issues that may need to be corrected - I don't think I've ever worked on a Q-Jet that only had a single problem...
Lars
In response to the OP's question about, "What is the adjustment?" - There is no one, set adjustment on the mixture screws. A Q-Jet, depending on year, carb number, and setup can have a "correct" idle mixture screw setting anywhere from 3/4 to 9 turns out on the screws from their seated position. Even 2 carbs with the same carb number will have different setting requirements. Without use of an air/fuel gauge, the easiest and quickest setup method is as outlined in your GM Service Manual, known as the "idle lean drop" method. With both screws set to the same setting, adjust them evenly to obtain best quality idle. Once achieved, screw both in the same amount to obtain a 50 rpm drop in idle speed. This is the factory recommended setting for best emissions. For best throttle response and operation, as Charlie noted, a slightly richer position is desirable. So, from the 50 rpm lean-drop position, simply back both screws out about 1/4 turn to allow the rpm to return to best idle position. This, of course, assumes that your carb is otherwise correctly set up and in good operating condition to allow the idle system to function correctly. Most carbs, if running poorly, have multiple problems and issues that may need to be corrected - I don't think I've ever worked on a Q-Jet that only had a single problem...
Lars
