My 440 rebuild thread

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Only thing I can think is, if it has to do with the lifters is they could be hanging valves open causing that pop, but that's just a semi educated guess.
Or your carbs need tuned
I don’t know if you’ve seen my thread Misfire at cruise, popping/afterfire at 3000RPM but there might be some details that a fresh set of eyes could pick up on. Some suggestions so far have been checking ignition, which I’ve partly done, and the possibility of sticking valves as you said.

One thing that keeps catching my eye is that #6 and 7 plugs are getting carbon fouled even after changing plugs. I haven’t checked the plug wires for continuity yet, will do that tomorrow.