The rpm at idle is about 500 according to an electrical meter.
The vacuum is about 20 inches of mercury, with a slight fluctuation.
No leaks can be seen or heard near the manifold - the audio recording was taken at the tail pipe.
Reving the engine does not take away the problem. It only seems to run smoothly when under load - of course then I'm in the cab instead of listening at the tail pipe, but it seems smooth.
I already have it advanced further than the factory settings, advancing more, even all the way to knocking, does no good. If anything, running it on the more advanced side causes the problem to be slightly worse.
When it is retarded, it makes a fairly constant, spitting sound from the tail pipe, and the truck loses most of it power when driven like that. I can not find a distributor position which gives a nice smooth idle without a popping sound in the tailpipe, though its a possibility I'm missing it if the region is extremely narrow.
