I am stumped. New bearings, sleeve and piston last year. Run 10% Byrons, 12x6 prop. Engine wont keep running at any needle valve setting. Not overly hot, runs about three or four minutes and quits. Tank even with needle valve, all new lines, clunk not stuck, blew out needle valve, new OS8 plug. No kinks in line anywhere. Sags at high and medium speed under load. Starts easily. Restarts easily as well-- just wont stay running. Have tried to richen, lean, neither helps. Any ideas before I send it back?

PLEASE!

Forgot-- new O ring on needle valve. Engine ran fine till about three tanks ago.