[Garryese]

Keith, your friends machine likely needs the idle mixture screw turned out 1/4 of a turn. Run the warmed up machine a few times. If the backfiring continues then repeat the 1/4 turn until it quits. If the total adjustment is more than 1 turn complete turn then the pilot jet needs cleaning or the float level needs adjustment.

On yours, I would pull the spark plug and check the color of the insulator. If it is white or light grey then you are running lean, which makes more power, but too much heat. Ideal color is light brown or tan. If the insulator is black then your rich, but I would leave it alone as you are not suffering from lack of power and the rich condition will help the engine run cooler, thus avoiding the Griz 660 studdering problems.

Keep in mind that some engines just make a bit more power than others. My stock non HO Sportsman 500 beats every stock 500HO sportsman I have raced; my engine is tight where it needs to be and loose where it needs to be.

The flashing light does not put the machine in a limp mode so don't sweat it too bad if the spark plug color is good. The light indicates you are hitting the red line, but these engines can handle more than the red line if they are loaded and it is not for long period of time.