Just bought a 93 Honda CB400 SF Pro. Big-1 with a little over 30,000km. It has been sitting for two years without starting so now when I start it smokes quite a bit. I haven't ridden it yet so maybe it will clear up but a mechanic friend of mine says that because it's nearly twenty years old, that the oil rings and valve seals could be bad. If anyone could provide their thoughts of knowledge I would be forever grateful.

He may be right, but not necessarily. My NC31 is 20 years old with similar mileage (kilometreage?) and it runs like a sewing machine and never smokes. It could just be the rings a bit gummed up with standing idle for a long time. If it does not clear up with some use, and still smokes, a couple of basic tests might determine where the problem lies.

Assuming its a blue oily smoke? Black smoke indicates rich running, white smoke is water vapour which could just be condensation being burnt off, but could also be a blown head gasket.

If you get a little puff of blue smoke as you change gear, or as you close the throttle or on the overrun downhill, it could be the valve stem seals allowing oil to be sucked down the valve guides by the vacuum in the inlet tracts. If, on the other other hand, it smokes when you accelerate hard it's more likely to be stuck or broken rings, or worse, worn bores/rings/pistons. Also check the plugs to see if they look oily.

Thanks so much for the info/advice. I've had the plugs, oil and air filters, coolant, battery and fork seals replaced so now I'm just going to ride it for a while and see what happens. After all, it's a Honda! Should run forever. Thanks again.