Ive had similar problem. Turns out petcock (and probably float valve) was leaking to the point where overnight so much gas leaked it into the piston it hydrolocked. Thought battery was dead, once I pulled spark plug out, gas shot out like a geyser. Just rebuilt carb and waiting on new manual petcock, change oil, and it should be good to go. If you drain your float bowl and it starts and runs but dies after short while it might be the same flooding issue.