More Auto 101 questions....Idle Speed

[Dennis Mosley]

When a carburated engine (no Choke) gets warm does it tend to idle slower?

Reason for question:

I have a rochester carb system on my car and it does not have a choke installed. When it is first started it seems to idle a little high but nothing too wild but after driving a few miles it wants to idle at a much lower rate. It hasn't died on me or anything but I think if I didn't give it a little gas while I am waiting at the traffic light it would.

[Bob Putnam]

Dennis,

If anything, the engine should idle faster when hot. I suspect that your idle mixture is too rich, so it's running better cold and stumbling when hot. You're probably fouling plugs too. Set the idle mixture when the car is fully warmed up. It might not run quite as well when cold but you'll have fewer problems in the long run.

If you have problems setting the idle mixture, check for vacuum leaks.