Motorcycle Repair: Honda 1982 185s ATC, market muffler, stock muffler

Question
I just installed a after market muffler and the header is stock from the exhaust port and then steps up after bend to fit the header which seems to have been made for the bike after market back in the day. Sounds great and runs real strong but now backfires on deceleration. Maybe jet problem? Any suggestions? Thanks

Answer
Matt, probably your aftermarket muffler is an open core unit. Can you see all the way through it? If so, it will be difficult to keep the backfiring down. The baffles in the stock muffler help to suppress the backfiring which is due to a pressure wave being reflected back from the open end of the muffler. When you cut the throttle off, the engine goes momentarily VERY rich, as the high vacuum generated in the intake tract will pull fuel down the idle circuits and even around the needle jet, which is collected in the header pipe and then ignited when the next hot exhaust charge comes through. Try to lean out the idle mixture screws as much as possible, without creating a rough idle or bog on transition to part throttle again.  Read the spark plug for signs of being too rich and jet it down to where it pulls well, but minimizes the backfiring.

As always, make sure that the ignition timing is correct, the valves are adjusted and you have good compression in the motor.

Bill Silver