I agree with Janicholson.
One other thing I can add from experience is the gas. It can put a film on the sparkplugs and make the back fire. Even if the plugs are new and run a short time. I have sand blasted sparkplugs before then blew them out real good with air then reused them and it takes care of the backfire for a short time. But the best cure in this case is 104+ Octane boost used as directed cures the backfire that I'm speaking of. I pick the octane boost up at Walmart.