Got this engine from my Uncle recently. It's a 1973 model engine but it runs great, if it would stay running. I just rebuilt the whole carb, replaced the tank gasket, diaphgram, etc. It will fire up, then run for a while then die out. Sometimes it will stay running on choke. Like I said, I cleaned out the whole carb. It has a new gas tank, and the fuel pick-up tubes are clean with no clogs. Help???