c3 Q jet
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Racer
Joined: May 2009
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c3 Q jet
Whats the best way to adjust fuel mixture on a 68 Q jet? I have seen a variety of procedures using a vacuum gauge, RPM gauge, even just listening for the smoothest idle. Some say you need to go back and forth between the mixture screws while adjusting others say just do it once each side. one said add an extra turn after finding optimal vacuum gauge adjustment. There has got to be the correct way to do this.
Last edited by greggome; Dec 18, 2018 at 10:33 PM.
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From: At my Bar drinking and wrenching in Lafayette Colorado
Greg -
I'm assuming you are actually asking about adjustment of idle mixture and not fuel mixture of the carb - there is no "adjustment" for overall fuel mixture.
First of all, you don't adjust one first and then the other - this results in grossly misbalanced idle mixtures. Both screws should be set to the same number of turns out from seated, and they should be adjusted equally, 1/2 turn at a time, from the initial setting.
The best way to set idle mixture is by using a wideband oxygen sensor in the exhaust and setting idle mixture to produce 14.7:1 (14.4 - 15.0 is a good range - I like it just a touch on the rich side to prevent off idle tip-in stumble) at the correct idle speed. In "drive" if automatic. Idle mixture will change with idle speed and manifold vacuum, so make sure that timing has been set correctly, and that vacuum advance is hooked up. Adjust idle speed to desired setting.
Vacuum gauge and tach are useless, except to get things "in the ballpark," as they will produce the same reading through a wide range of idle mixture from 13:1 to 15:1.
Without a wideband, the best method is as outlined in the GM Service Manual for your car by using the "lean drop" method: Adjusting both screws equally both in and/or out, find the spot that produces best quality idle by ear. With a tach hooked up, then screw the screws both in equally, 1/2 turn at a time, until a 50 rpm idle speed drop is achieved. This will produce idle for best emissions, but it will be a tad lean for my likes. Once this point is achieved, you can then back both screws out 1/4 to 1/2 turn to obtain best idle.
Note that this applies to Q-Jets only and not the Holley-type carbs. The idle mixture screws on the Holley-based carbs are much more sensitive than the Q-Jet carbs, and a 1/8 turn change will make a dramatic difference in idle fuel mixture.
Lars
I'm assuming you are actually asking about adjustment of idle mixture and not fuel mixture of the carb - there is no "adjustment" for overall fuel mixture.
First of all, you don't adjust one first and then the other - this results in grossly misbalanced idle mixtures. Both screws should be set to the same number of turns out from seated, and they should be adjusted equally, 1/2 turn at a time, from the initial setting.
The best way to set idle mixture is by using a wideband oxygen sensor in the exhaust and setting idle mixture to produce 14.7:1 (14.4 - 15.0 is a good range - I like it just a touch on the rich side to prevent off idle tip-in stumble) at the correct idle speed. In "drive" if automatic. Idle mixture will change with idle speed and manifold vacuum, so make sure that timing has been set correctly, and that vacuum advance is hooked up. Adjust idle speed to desired setting.
Vacuum gauge and tach are useless, except to get things "in the ballpark," as they will produce the same reading through a wide range of idle mixture from 13:1 to 15:1.
Without a wideband, the best method is as outlined in the GM Service Manual for your car by using the "lean drop" method: Adjusting both screws equally both in and/or out, find the spot that produces best quality idle by ear. With a tach hooked up, then screw the screws both in equally, 1/2 turn at a time, until a 50 rpm idle speed drop is achieved. This will produce idle for best emissions, but it will be a tad lean for my likes. Once this point is achieved, you can then back both screws out 1/4 to 1/2 turn to obtain best idle.
Note that this applies to Q-Jets only and not the Holley-type carbs. The idle mixture screws on the Holley-based carbs are much more sensitive than the Q-Jet carbs, and a 1/8 turn change will make a dramatic difference in idle fuel mixture.
Lars
Last edited by lars; Dec 19, 2018 at 03:29 PM.
