Re: 5.0 Mercruiser Bayliner won't idle - Help!
It is a carbureted engine, idle speed is set at 650. Idle mixture too lean? PO? Can you explain? Thanks.
PO stands for previous owner.
650 is about right, so that's probably not the problem.
Carburetors have two main fuel delivery circuits. One is the idle system, which basically allows a fixed amount of air and a fixed amount of fuel into the engine. You can adjust the amount of air with the idle speed setting and the amount of fuel with the idle mixture screw. As you raise the throttle, the engine pulls enough air through the carb that it's able to meter effectively through the venturi and the fuel is pulled through the jet into the air stream. The idle screws are still admitting fuel into the engine also, but the amount is very small compared to the amount going through the jets.
Sometimes when the idle isn't right, somebody that doesn't know what they are doing will raise the idle so it won't die on them. Raising the idle speed without adjusting the mixture screws makes the idle mixture very lean, so they keep raising the idle until the engine is actually pulling enough air in that it's coming through the jets and normal fuel metering. It's no longer relying only on the idle circuit to feed the engine.
What would happen if this were the case is that as you shut the engine down it might run lean for a moment and the engine would die under sharp deceleration. But, you'd have to have the idle at 1000 RPM or so to do that, or you would have idling issues now. I assume you don't have any?
