Im working on a 6.0hp craftsman walk behind, when I let go of the dead man handle it "thumps" a few times and then stops. The thumps shear off the fly wheel key and last time tore the blade adaptor key off. The shaft is strain and the blade is balanced. Its ripped of the flywheel key 3 or 4 times now. I dont know what else to try. Any ideas? it's a Tecumseh engine. thanks