Adjusting Idle Mixture Screws Does Nothin - Quadrajet
 

after rebuilding my Rochester Quadrajet 4MV I wanted to adjust the idle mixture screws, YouTube says to adjust by RPM or vacuum, well nothing happens when I screw the screws in or out, no change in RPM speed

what could it be?

it could be a poorly adjusted choke

it could be a vacuum leak

anything in particular I should check and how

1974 GMC 1/2 ton manual 350 V8 all stock

Re: Adjusting Idle Mixture Screws Does Nothin - Quadrajet
 

If it's idling too fast you can't set idle mixture. Set it when the engine is warm and at low idle in the 600-750 range. You should be able to see the vacuum gauge change position as you adjust.

Re: Adjusting Idle Mixture Screws Does Nothin - Quadrajet
 

If they are not responding, you more than likely have a vacuum leak. Check that all gaskets are sealing properly by running the engine and spraying carb cleaner around the base. If the engine speeds up, you have a vacuum leak.

Re: Adjusting Idle Mixture Screws Does Nothin - Quadrajet
 

The motor needs to be fully warmed up before adjusting the idle mixture. Choke fully open. Idle between 600-700 rpm. Vacuum should read between 18-22 if things are close depending on if your distributor vacuum advance is on ported or manifold vacuum. Timing with no vacuum advance should be around 8-10 BTC to start with.

If vacuum is way low at idle, suspect a leak or poor timing.

Before starting, screw the mixture screws full in until they gently seat. Too hard and you can enlarge the hole in the carb. Note how many turns each is out. Then back them out 1 turn and start adjusting. One turn out should be too lean but this way, you only have one way to adjust. At some point around 1 and 1/2 to 2 turns out, the motor should speed up a bit. Further turning out won't yield more speed. Find the best idle and vacuum reading, then turn that screw in 1/4 of a turn. The motor will slow a bit. Repeat for the other side.

Re: Adjusting Idle Mixture Screws Does Nothin - Quadrajet
 

Thank you all for your advice, I drove the truck in to work today for the first time, and after I drove it back home (10 miles) it was finally warm enough to where after screwing each side in one at a time the truck finally responded and the motor died

Then I turned each screw 2 full turns out from bottoming out and learned that its around 2 full turns that the RPMs are at their highest turning the screw out any further yields no better results, as you also suggested

I tried turning the screws in a bit after that to find that sweet spot but I think the best results were at 2 full turns out

so thank you all for your help, I consider this job done

Re: Adjusting Idle Mixture Screws Does Nothin - Quadrajet
 

The best way to set idle/mixture is by a vacuum reading. You adjust them one at a time until you achieve highest vacuum reading. Attach the gauge to any manifold vacuum source.