New to the karting world. Got my son into a stock engine class on a 1/4 mile track. We have had all sorts of problems. We run a 12 tooth clutch, 35 chain, 60 tooth sprocket. We have unhooked the governor but not taken it out. We have had some 19-20 sec lap times but as the night goes on the hotter it gets the more problems we have had. Karts would go 6-8 laps then die. Start right back up and go again for about same amount. We thought the mufflers were over heating the tanks so added straight pipes. Worked for a week or so then back. Brand new motors out of box doing it. Thought has tank was vapor locking so removed gasket on gas cap. Thought ethanol in gas was issue so tried race fuel. Any ideas?