Could it be the mixture is a bag out ? I tinkered with mine and that did appear to have an impact on backfires, albeit at low revs.........just a thought
mine did this, checked everthing turned out to be a dodgy electronic ignition. not sure if you run one. have you got a spare points and condensor or possibly another dizzy you could just pop on to test and cross off the list?
Valve timing? I had loads of trouble with mine and alot of it turned out to be tappets a bit too tight.
Tried 3 sperate dizzys with P&C and electronic ignition. Carb came from the old engine and has been setup correctly.
The sound you describe; backfire as in fuel burning in a hot exhaust. Or. Detonation, fuel air mixture going pop either too late (valve open) or as a result of too lean a mixture? In the case of the former, assume the carb's fine, have you checked fuel delivery at the pump? Failing that, fuel pressure too high, or sticking float? What is the timing set to at max advance circa 3k rpm? In the case of the latter I'd go back to your earlier thought around camshafts. Before that though, one last but unlikely easy fix:- pushrods correctly seated, or rocker assembly incorrectly seated? When building the engine if the Cam was seriously damaged this would be visible in the form of pitting or missing surfacing from the lobes, so I'd doubt you'd miss that. One other thing I've seen before is the distributor drive shaft in 180 degrees out. Engine would run, but not at all well. Your's on the bench I take it? Pedro!
