Small Engines (Lawn Mowers, etc.): exhaust backfire

Question
tecumseh hm100 in generac generator. fire out exhaust. ran some Marvel oil , settled down alot but still gets some fire out of exhaust. eng family ytpxs.3582ba   159386a. ideas? Thank You

Answer
Could be a lean carburetor, ignition timing depending on if it has points or not, or a mechanical timing issue if the valves are not adjusted properly.

It could even be excessive carbon build-up on the exhaust valve not allowing it to fully close.

Can you use the generator as it is?  If not, how much money and time do you want to invest in it?

Eric