Hi guys, our 4 stroke 110 a/c engine will backfire after having the throttle at mid or higher and then letting it back off? What can be causing that?

the engine ignition timing is fixed so it cant be that and I cant see how the engine timing could have changed.Could it be a too rich carb setting?

 

A lean condition will cause that... also could be caused by a leak where the head pipe mounts to the engine..

 

Running too lean, most of these chinese bikes/quads come like that. You can try changing the needle clip postion, adjusting the air fuel mixture and swapping the main jet for a larger one.

 

Yea,I thought it might be too lean as well,but the plug is really black,and you can smell how rich it is burning.I am stumped at this point.

 

To get a good plug reading checking the jet (main jet) that would be involved in a high speed back fire..... ride the machine at full throttle.. leaving the throttle pegged.. hit the kill button....coast to a stop... then pull the plug.. if you let off the throttle at any point before killing the engine.. the reading is flawed..