My Vette is running rich, and I need to adjust the mixture screws. I have not had to do that in years. Can someone give a basic how to on adjusting these screws????

Thanks in advance!!

 

In is lean and out is rich. Mixture screws only affect idle - not cruise, so the screws will not change anything if the car is overall running rich. Start with a mixture screw setting of 4 turns out from lightly seated and keep the screws evenly adjusted. Set timing correctly before adjusting mixture.

 

I kind of remember that I used to turn each screw slowly until the idle rpms stopped climbing. Is this not correct? The timing is dead on. How would the timing affect the mixture??

 

Timing affects idle speed, and thus has an effect on the resultant throttle blade angle. Varying the throttle blade angle (idle speed) changes how much of the transition slot is exposed at idle, and this alters the amount of fuel that is discharged at idle. Increasing blade angle (forcing the idle speed screw in to compensate for inadequate timing at idle) exposes more transition slot, and richens the mixture, making the idle mixture screws less effective. In excess, it will make the idle mixture screws completely inoperable. I always tell people that "90% of all carb problems are timing problems." Make sure timing is right, make sure the vacuum advance is operable and hooked up right, set the idle speed right, and finally adjust the idle mixture. Once done, re-set idle speed for the new mixture setting.

Start at 4 turns out. Adjust the screws 1/2 turn at a time equally either in or out (depending on the engine's requirements) until best idle quality is achieved. Then screw them both in 1/4 turn to produce what is known as "lean drop." This assures that you're slightly on the lean side for good emissions. If emissions is not a concern, leave the screws at best quality idle. Screwing the screws may or may not produce an increase in idle speed, but you should be able to sense a change in idle quality.

As a rule of thumb, a 1975-1980 Q-Jet should be at about 4 turns out, and the 1974 and earlier carbs run at about 2 turns out. If your 1975-1980 carb runs best at less than 2 turns or more than 6 turns, you usually have a problem with the carb. Likewise with the pre-'75 carbs: If the carb runs best at less than 1 turn or more than 5 turns, there is usually a problem.

 

Hey Lars......I don't want to get this post sidetracked since I had one started........take a look at my "Me again post".......T.Y.