I have a Tecumseh HM80 on a Troy Bilt Horse tiller. It is the second engine installed about 10 years ago. It probably has 75 hours on it. With clean fuel, correct clean 30w oil to proper level and clean crankcase breather, the engine runs fine with no exhaust smoke for 15 or so minutes then I get a huge cloud of white smoke that could hide a house. I have seen discussions of this on the net and people usually say "worn engine". If this were a worn rings-bad valves issue, wouldn't the engine smoke all the time and wouldn't the smoke be blue? Anybody have any thoughts on this?