Join Date: Nov 1999
Location: Central California, USA
Sounds like you have a cylinder that's backfiring every once and a while. You could have a vaccum leak from a bad sealing intake gasket, that would make a lean condition on that cylinder mainly at idle.
I've also seen leaky vaccum hoses do the same thing. Some power brake hoses connect to a vaccum port that mainly goes to one cylinder, making it lean at idle.
Spray some window cleaner or water around the intake and carb when the engine is idling and see if things change. Try clamping off vaccum hoses, one by one.
The intake leak could be coming from the valley side of the manifold where you can't see it. An engine that has been decked with milled heads, will be more prone to manifold gasket problems.
The older I get, the faster I was!