It sounds like you may have the Hi speed needle too lean or the fuel is restricked some place in fuel line or carb. Even a air leak where the carb attaches to engine. Does the engine Idle good and stay running. If so, when it is running...squirt a shot od WD40 spray on all gaskets by carb. If the RPM changes when you do this...you have air leak. Check this and let us know. Capt,n