I'm working on a Troy-Bilt TB575 EC 4-stroke string trimmer for a neighbor. It almost jerks the rope out of my hand when I try to start it! I've already adjusted the valves. I've dealt with lawn mowers that had sheared flywheel keys, so I thought that maybe that was the issue. I checked the parts diagram, and Ii don't see a flywheel key! Is there some sort of timing that I may need to set? Thanks, in advance, for the help!