I have an '83 Honda XL600R that I use for commuting, running errands and some fire-roading. When it's cold and first started, it produces great billowing clouds of blue smoke, especially when the oil tank is full to the top. I could swear that I read in one of the magazines some years ago that these bikes have a check valve in the oil system somewhere which can leak or go bad and cause this problem. My Clymer shop manual doesn't show it. I will be doing the top-end on the bike this winter and would like to fix this. Does anyone know about this valve, and where it might be?