carb is running rich

by robert

how do i adjust air-fuel ratio and idle?

Comments for carb is running rich

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Sep 02, 2012
by: Anonymous

It would sure help to know what year/carb you have. Assuming you have a C3 you most likely have a Rochester Quadrajet. The most common cause of rich conditions are 1) choke does not open fully when engine is hot, 2) the float in most Q-Jets are made of a special plastic that, over time, can absorb liquid fuel causing it to sink in the float bowl and allowing the fuel level to raise so high raw fuel dumps into the engine. 3) worn needle and seat in float bowl. 4) power valve for main metering rods stuck in the up position with engine running. The power piston controls the position of the metering rods to control fuel flow through the main jets. Old gas, dirt, and varnish from cheap fuels can build up on the piston and it will not move down when engine is running, allowing full rich fuel flow through the jets at all engine speeds. Idle adjustment is simple, but you must be certain you have no vacuum leaks at either the base of the carb, any vacuum lines, (there are alot of these on a C3), EGR valve is not sticking open, bad PCV valve or hose. These all affect idle. If your car is a pre '79, there will be plastic anti-tamper caps on the idle mixture screws located in the front of the carb near the manifold side. After '79 these screws are capped by steel plugs and require a bit of work to remove and a beginner should not try that. Once you have removed the plastic covers ( they will break easily by now) turn both mixture screws in until lightly seated. Then counting the turns, back each one out 3 1/2 turns as a starting point. Run your engine until warm. To see if you are rich or lean at this point at idle remove a vacuum hose, the one to the rear of the carb that works the headlights comes off easy. If the engine wants to stall and runs rough, you are lean, if it doesnt change or speeds up slightly, you are rich. You want the fewest turns out of the screws without being rich. But do not adjust your base idle mixture until you have fixed your rich condition, you will be wasting your time.

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