Smoked On Startup At My Race Sat Night

I have a 99 yz400f and went to my first race(on the yz) and made my practice run and then went to fire it up for my first moto it smoked like a train for a few long seconds.My problem is I cannot get it to do it again. I have searched this problem and like in automotive the valve seats start to leak.The thing that confuses me is I cant get it to do it again...Do you all more knowledgeable people agree that the problem lies in the valve seals or seats? I have more know how to the 2-stroke scene and have never really messed with valves. My manual states that I need a few tools to do a valve job. I am kinda in the dark on this one I just hope it is the seals leaking off a little oil into my jug. If it helps I have already checked oil for water and water for oil and all is clean. Can someone help me find my culprit? If I know for sure that is what it is then Im pretty sure I can handle doing the work. Someone with some know how PLEASE HELP...It scares the life out of me seeing it smoke the other night and would love to get this fixed :thumbsup:

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