I have looked and looked for a vacuum leak and I just can't see anywhere it could be. The manifold is plumbed for vacuum. I hope there isn't a leak under there somewhere!
I took the dizzy apart last night and it has 26 mechanical built in. I am going to sften the springs and reduce the mechanical to around 15 total so I'm all in by 2000 to 2300 and I can set initial at 20. I don't think that is the problem either but I wanted to change this anyway.
Back to the issue. The webers should be able to run at 12.5/13 to 1 a/f without popping and it is only popping on one side. If it leaks where the header meets the sidepipe, would that cause this or is it to far down stream? If the header is tight on the head and there is no sign of black sought, where else can I look?