I would call Aircraft Int. and ask them for an opinion. I have the same engine with no problems at all but have found Gerhard very willing to help with minor problems or questions. Worst case, the engine must still be under warranty I would guess so send it to them to get checked out.

Back flow from the exhaust is caused when your outlet pipe is to small or in the case of a weed eater muffler, the baffles inside get built up with so much crud that it seals off your exhaust flow. I rebuilt a weed eater carb once only to discover that the muffler was packed with gunk which limited the rpm of the engine severly at anything past half throttle. I doubt your muffler could be clogged this soon.