99 400ex cooling fan

Hi guys. I have a problem where the quad will start and run well for about 5 minutes and then it dies. On inspection the airbox has oil in it (quite a lot). The oil is pumping through the cylinder head breather hose. This problem first happened suddenly. I have since done the top end including, piston, gaskets, rings and valves. I have also done the carb (not that I thought that this was the problem). The compression seems normal. The only thing that I can think of, is that I did not grind the valves well enough. Anyone experienced this? If so, how did you fix it? Any other thoughts?

What should I do about this , bad oil leak where the filter cover bolts on . Can I use some gasket maker 

My friends KQ 300 starts fine and runs, as soon as you put it in gear and try to accelerate its bucks and farts like crazy, not moving... I cleaned carb and it worked good for 1 day... now back to same problem.... could it be such as a bent valve????

I have been exploring all the options available, so many, its hard to choose. I have kind of narrowed it down to the Honda Pilot, Subaru Outback, and Kind of looking at the Outlander, although its a little small,  the new one is nice looking. Just wondering if anyone has any recommendations or gripes with any of these rides. I have driven Jeeps for the past 25  years and they have been great, but their price point has gotten to high. Expieriences good or bad would be helpful. Thanks!

I’m looking for a top end kit for a Yamaha. I would like to see how well the engine runs before spending $6-$7 hundred on oem. Niche has complete kits from $1-$2 hundred.  Anyone have experience with Niche or similar Chinese companies?