Dan most of the time a 4-stroke backfires it's to lean. On break in for the first few tanks it should be so rich
it will die without the glow starter on the plug. If you don't have a tach get one. That engine should run
6,000 or less for the first 2 tanks. Then less than 8,000 for the next two. I would not run it more than
8,000 for the first 7-8 tanks of fuel. Also , what prop are you using. You should be running an APC 13*6.
let us know what you figure out. later daveo