Hello all,
New member here. I have a bone stock 2004 Grizzly 660 that I had some sputtering issues with this year. I cleaned the carb and set the fuel level , but that did not fix my issue. Whenever I would go slow up a hill or over a log it would sputter between 1/4 and 1/2 throttle. I installed a Dynojet stage one kit and rechecked the fuel level which seems to have solved that problem. Bike runs strong and has better throttle response, but I have a different issue when riding hard. If I am riding hard, speeding up and slowing down I will get a single exhaust backfire if I pin the throttle wide open from the closed position. No stumble in performance at all though. What would cause the single backfire from closed to wide open throttle ? It doesn't do it every time and not from a dead stop. I have 27" Mega Mayhems on it and it will stand up on the back wheels from a dead stop.

 

As ignition timing is fixed by the trigger coil and CDI it has to be carburation at fault, either weak or rich. Usually rich, mixture doesn't get completely burnt and some fuel goes out of the exhaust port into exhaust, when the next slug of still burning gas hits it, it catches fire, causing an explosion in the exhaust. A leaky exhaust also helps do this, as air is also needed for the gasses to catch fire. Honda 450s all backfire on the overrun and I have had to fit new silencers to quite a few, as they blow the baffles out. Had one last week in fact, but silencers are on back order for them.