9hp techumseuh
It backfires when accelerating or decelerating and I can't figure it out. What would cause something that?

water in your gas, flywheel key marked just enough to throw the timing off, a valve not seating right, a valve guide thats worn out

Hey, how old is it??If its a newer engine 1-5 years it usually is a carb problem, where as older engines lean towards wear and tear. Checking the fly wheel key is a good thing to get out of the way, but you are problably looking at a valve job. Do it yourself and your looking at $15-25.00 to have the valves reground, and buy a valve grinding stick and some valve lapping compound. If this is over your head, most repair shops will stick you for $90-150.00 but will most likely do the valve seats too. Ya get what ya pay for, sometimes

2 years old, the previous owner attached some custom muffler would that cause a problem. Like with back pressure on the engine. The engine seems to new to have valve problems

its possible, but it might just be the way the muffler is made