Could be oil starvation damage to the rockers or cam,...
... but,... it's possible that it is bad jetting, causing detonation... or spark knock...
I'm suspicious of this because of all the popping and dying...and the fact that it takes 50 seconds to start making noise...
In olden times,... as cars were climbing mountains the engine would lug and get hot, and the air would get thin, and cars would begin to knock... The solution was to flip the air cleaner lid to allow more air and lean the mix...
I would clean the carb and jets and check what size jets are in there...
I always check the cheap easy fixes first... (with fingers crossed)...
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