I had a simillar problemwith my 1975 HS-50. Fox is right...a new plug did wonders. I still had problems though. Recheck your idle speed, clean out your fuel filters, and check your timimg. I did all that and it starts (and runs) fine. I did find that it starts WAy easier when warm, and when it had problems there, it was because the fuel flow was blocked. As for running with no air filter, you may be leaning out at higher speed. I found that if my performance was better with a little choke at full throttle, it was too lean. I hope this helps.