Motorcycle Repair: adjusting the timing, plug hole, airbox

Question
Thanks for your prompt reply concerning my 1991 Yamaha yfm250.  You thought the problem with the gas running into the airbox was timing related.  Lacking a shop manual, how would I check the timing and correct the problem?  Would this be something a real mechanic should attempt?

Answer
take off spark plug turn engine over . using a straw i pass it down the plug hole till the piston comes al the way to the top this is called top dead center or/tdc/.
then pull engine sider plate off look to see if timing marks are lined up with each pther o.k sorry about delay holodays hectic .

mitch