Bought a Stihl 291 at a pawn shop. It runs really great--except that when I start it cold, it tends to fire early (I think) and jerks the rope out of my hand. Then, when it fires and runs, it sounds like the coil shorts out a little, then pretty quickly smooths out and sounds real good. Is it the timing, or do I have a partially bad coil? I checked the key on the flywheel, and it doesn't look like it has been damaged or filed, so I can't say that it has been intentionally altered. Can a coil be going bad, then straighten out? My electrical experience says that if it's shorting out, it won't magically stop. Appreciate any help.