Okay, so I got this (new to me) 1980 Yamaha QT-50 the other day. I can't figure out why it won't start either. Sometimes it will start, and run for a couple seconds (with starter fluid or with premix). I have tested that the engine is getting both air and fuel, and I'm also getting a spark. I took the exhaust off to see if backpressure from a dirty exhaust was the problem, no go. I cleaned the carb, but there is a small pin that's stuck (I can't even get it out.). The large pin works fine though. Does anyone have any other ideas as to what I could do to get it running?