the more economical engines will usually do the backfire deal.
i say engine but in reality it is the tractor manufacturer that adds the muffler most times.
the same tractor can have many different engine options.
for example the huskies we sell come with kohlers and kawies.the kohler will always backfire and the kawies never do .same mufler with each engine and the kohler on these tractors has the solinoid on the carb and it still backfires.
its just the design...usually a large single cylinder engine .
you should run it at half throttle for 30 seconds or so and then shut it off.
later chris
craftsman 10/28 snowblower with tracks husky 372xpg chainsaw sachs dolmar bc212 bushsaw mondo trimmer monster tractor with trailer cheep wheelbarro and couple shovels and a partridge in a pear tree
