Chainsaw Repair

I think your timing is off with the 2 bolt coil?

The backfiring is the give away to a timing problem.

There's typically a problem with newer design coils being installed at the exact position of the old coil . . .

And the result is the timing is off due to a secondary winding having been added inside the newer coil to provide a timing advance with rpm.

If you could set the timing correctly by rotating the flywheel position relative to the key way . . . The saw should run fine?

This is accomplished with a degree wheel and timing light. The figure for the old fixed advance coils is between 20 & 25 degrees advanced.

The standard figure for advanced timing at 5000 rpm or higher is 28-30 degrees. But 28 degrees at starting will jerk on your fingers.

This is often done by chainsaw racing guys but rarely done at a Stihl Dealer repair shop.

All this is explained by others somewhere on this site in a thread.