Ok, this should be an easy one, for some of you. Have some older Stihl's that I have cleaned up and want to put back in service. I have removed engine and cleaned, new spark plug, fuel filter, new coil. I have removed piston to inspect; mild carbon on top of piston, rings look ok. Doesn't appear to be any scoring. Machine will start 3 to 4 pulls, and will run for about 8 to 15 min. depending on the working load, then will just die. They will restart when they cool. Any suggestions?