Yup a blown power valve will do that ( from a previous engine backfire)
On a blower motor that does not have enough idle timing the chance of a backfire is greatly increased.
It don;t get any better as the plugs foul, either.
Old stale gas is a pain. Ethanol or not. Lots of people try to blame everything including bad luck bad weather and more
on ethanol. Most of that is BS.