GMC 950 Popping through exhaust

[Deereman]

I have a 71 GMC 950 manure truck with a small block and 2 jet Rochester carb. When it gets up to speed going down the road it starts popping out the exhaust when cruising, if I give it more throttle it quits and starts doing it again when I back off the throttle. It seems to run fine except this no loss of power or rough running other than the popping. Any ideas? I messed with the timing a bit today and had it advanced to the point it would spark knock when starting out so I moved the timing back to where I started. The vacuum advance isn't hooked up I'm not sure who unhooked it, I did put a hose on the advance can on the distributor and sucked on it and didn't see anything inside move so I'm thinking the diaphragm is bad. 

Edited January 23, 2022 by Aaron Wilson