Postscript: the original symptom was typical of a blocked main jet. The engine runs fine when pulling through pilot and needle jets but above half throttle the main jet starts to suck and if it's blocked the engine will run leaner and leaner and stall. Often the main jet blocks first, even though it's the largest, because it sticks down into the floatbowl so any debris that drops in there gets sucked into the main first.
Glad it's sorted.
Re popping back in the exhaust the usual cause is an air leak at the joint between the exhaust and the cylinder head. With a rattly engine you may not hear a slight gas leak on throttle, but on over-run it sucks air back into the pipe and any unburnt fuel ignites, hence the popping. I've also heard of this being caused by the idle mixture screw on the carb being set too lean (same effect, too much air on closed throttle). Take the exhaust off, check carefully for leaks or splits then refit and tighten down carefully with a new gasket. Set the idle mixture screw per the handbook and that should do the job.
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