Working on a Stihl BG 56C leaf blower that stopped running. Wasn't sure if it was an old fuel problem, air filter, or spark plug problem. Ended up being none of the above! The exhaust was plugged up by mud wasps. I thought to look at the exhaust because every time I would take the spark plug out and put it back in it would sputter and then not fire again no matter what you did to it. Hope this helps!
