I have a John Deere LT150 with a Kohler Command 15hp motor. It starts fine and runs fine for about 10 minutes. it will then bog down, blow some whiteish smoke (not a whole bunch) and die in about 30 seconds. Friend said maybe valves. There is quite a bit of carbon build up on them (exhaust especially). I can"t see any other damage, they are not bent. They are tight in the guides. I will lap/grind the valve seats. I really need other suggestions of what to do while it is apart. Do y"all think it is the valves? Or something else? The cylinder wall is smooth and undamaged but I will do rings while it is apart. Give me any advice that y"all have. I know there is more knowledge here than between me and my friend. I really like this site, I just RARELY post. Read it alot though. Thanks.