Excess carbon in the combustion chamber or the muffler was coked up with carbon deposits.
That wasn't either. I actually did a decarbonization on it which didn't resolve the problem. BTW none of the local Stihl can even get the decarbonation fluid so going there you are SOL most times.
That is something I have been running into lately on both Stihl and Echo 2 cycles where carbon is locking up the cylinders. And spark arresters have always been a problem area so they get pulled and clean if needed on every unit.