The magazine for classic Japanese motorcycles

Q&A: Heavy smoker

The other day after a good ride, I noticed a few oil drops on the pavement just underneath the exhaust pipe exit. The bike generally runs very well, although it does not fire properly on the right cylinder for a few seconds after going down a steep hill in the neighbourhood with the throttle in the off-position. What could be the reason for this behaviour?

Jan Aasen
Stavanger, Norway

A: The misfire is probably caused by the spark plug temporarily being contaminated with excess oil. The first thing to do is simply to take the bike out for a fairly hard ride to see if that clears the problem. If there is no difference after this, the fault is almost certainly a worn or aged crankshaft seal which is allowing transmission oil to be sucked into the crankcase. Have you noticed the transmission oil level going down? Another possible test is to check if there is a difference in the exhaust gas smell between left and right as smoke from burning transmission oil smells different from that from burning two-stroke oil. If these tests do not show any signs of abnormality, I think your next step should be to check the compression pressures on both cylinders, as this will give you some idea of any wear which may have taken place. Because of the rather limited oil supply offered by a typical two-stroke engine, bore wear is generally quite rapid compared with a four-stroke.

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