I had a similar problem with my 351 build and a Model 4160 Holly square bore. It kept popping through the exhaust after warm up, and occasionally through the carb. Turned out to be the float in the primary side was allowing fuel to dripple into the carb because the fuel level was too high. I had adjusted the thing all the way down, but it was till at the top of the view port. Had to bend the brass tab on the float. I have no idea how it got knocked out of adjustment.

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Mountain Motor Rider