I have an issue with my gen 4.
I bought the bike used and since I bought it, Its been leaking oil from the airbox. A lot of oil in less than 10 miles.
Unfortunaly the guy I bought it from wont take the bike back.
I did a compression test and it shows 210 in all cylinders. What would be the next step to follow.
One of the reasons for oil in air box is a blow-by, and that is rings.
How many miles on the bike? have you started the bike with the air box open? and can you see the blow -by gasses?. if so you need to do a leak down test and if there is more than 10% lost, you need to rebuild the engine. ( new Rings)
Sometimes if the bike is tipped over that oil can run up to the air box , have you tried to clean it and see if it would get oil back in the box again?
block off plates installed? if one or both of the reed valves under the block off plates is installed upside down it will pressurize the heck out of the crankcase and blow a lot of oil in the box. i run all bikes just below the top of the sight glass standing bike straight up.
...to add, there is usually a little filter material in the air box where the crankcase breather connects... I'd check that for obstructions or being overly wet. If it is, the crankcase will not vent vapors properly and will actually start pumping fluid up through the breather instead of vapor once the restriction gets to a certain point.
Have you taken the airbox apart yet and cleaned it up? It is normal for a little oil to collect in there from the breather, but once they fill up enough to start pouring out on their own it can make the problem seem a lot worse than it is. If you haven't taken the box out completely do that and clean it, dry the coarse filter, and then see if it 'leaks' after that.