New to this forum, so be kind please. I have a tecumseh motor on my weedeater push mower and it has occasionally knocked some and would not start the other day. It pops and kicksback when you try to pull start it. I found that the flywheel keyway was broken and replaced it with the correct manufacturer part and thought that would cure the problem. But it still does the same thing. Any ideas? Valves may be bad? or what?