I have been trying to get my 300ex to start for sometime. a few days i got it to start over and over and then the next day it wont start. I do the value lash and keep the covers off. I was (prob exhaust) smoke coming out of the covers with for the valves. I also noticed that the crank position marker had smoke also coming out, but i just thought it was from the head and it being in the cold. The hole for the position marker also has air come out when it turns over, but it is very light of air flow.

 

I also felt air coming out of the valve cover
here is a video where you can kinda see the smoke -

 

Typically crank cases are vented back into the intake , then burned. Way back in the old days, crank cases were just vented into the atmosphere. Here is how I vent the crank case in my Arctic Cat 700 Diesel after installing a turbocharger. Ran a hose up into the Black Boar front box into a "Vented Catch Can" Yours should be able to vent into the air box.

 

when i got the atv i was not given the air box so it is just out in the air, i think it could be burning some oil tho because it has white smoke and when its off it has a lingering thick smoke at the tip.