There have been similar posts to this problem but not exactly the same. This is on a grasshopper mower with a Kohler Command 18 HP horizontal shaft engine. It isn't mine either, it is a friends. Background info. At the end of last year, somehow the valve seat insert came loose (not sure which valve). So, in February, he just bought all new parts. New head, valves, rockers, and push rods. Reused the same lifters. Keep in mind that this is only on one side of the V-twin, he did not even touch the other side. New gaskets and seals. He put everything together and the machine ran great. Then about a month ago, it started using a lot of oil, about 2 quarts every 6 hours. Then about 2 weeks ago he went to mow and when he put the machine under load, it died out. When he restarted it, it ran fine at an idle except it smoked like crazy. He then noticed that oil was oozing out of the exhaust where the short tube bolts up to the head (right through the gasket). The first thing I thought was to check the breather. Well, I actaully haven't pulled it out yet, but I looked in the air intake just above the carburetor and there is a little bit of oil there, but not much. Not enough to look like 2 quarts went through it in 6 hours anyways. In addition, the leak is only coming out of the side that he rebuilt the head. The other side is dry. I also pulled the plugs and the side that he did not touch, the plug looks normal. It's dry, clean, and looks good. The side he rebuilt however is a different story. It was caked up so much with what I assume is carbon (it isn't all black, it has some grayish color to it mixed in with black carbon) that you couldn't even see the electrode on the plug. It was just one big glob of hard crusty substance. So, I am guessing that the breayther is not the problem. Otherwise both sides should have shown signs. The next thing I thought was that they damaged the head gasket while instaklling the head. So I took the head back off and was amazed at how much buildup there was on the piston and the head. I bet the buildup on the piston is about 1/4 inch thick. It's really bad. I removed the valves from the head, and the back of the intake valve (on the head, not up the stem) has burnt oil accumulated on it. The exhaust valve also has some buildup but it seems lighter and more completely burnt, which makes sense. The head gasket did not show any signs of damage. I also measured the head out and there is not an excessive amount of clearance between the valve stem and guide (about .001") which I didn't figure there would be since they were both new in February. The oil seal on the intake valve also appears OK, no visual signs of damage and seems to have a good fit on the valve stem and a real good grip on the guide that holds it in place. So my question(s) is...Have I exhausted all possibilities on the top end? Is it still possible that the oil is coming in through the intake, breather, valves, etc? Or, should I clean everything up, put it back together, and then perform cylinder compression tests, leak down tests, etc? (I know I probably should have done that in the first place, I just started assuming stuff once he told me he rebuilt the head this past winter).