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GMC 950 Popping through exhaust


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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 by Aaron Wilson
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