Synthetic oil has damaged seals

    I changed to fully synthetic oil recently. Over the next few thousand km the engine began to smoke at start up whether parked on the side or centre stand. I suspect this was the valve guide oil seals. The engine has also smelled fumey. The rear seal has also given way and oil has been leaking through the weep hole in the bell housing and my clutch has been slipping. My engine had no leaks before this and has only done 150,000km...it never used any oil other than what I would take out.


    I changed the oil for fresh synthetic and used an engine stop leak additive. This is meant to rejuvenate the rubber seals. I haven't gone far since... the weep hole is still leaking but I might have noticed a slight reduction in smoke at start up.


    Can anyone suggest a longer term plan?

    This problem is caused by the synthoil, but it's not the fault fo the synth.


    Basically, after 150.000 your seals where damaged, shrunk, broken, dry etc. anyways.
    But the mineralic oil you've been using before tends to leave residues over time, which act as additional sealing. Therefore your engine did not leak before.
    The synthoil washed away those residues, hence you're leaking now.


    I am afraid the only correct answer to "what to do" is: replace the seals. Not overly expensive if you can do it yourself, or friends can do. But a bit of work.

    My K75 used to run on fully synthetic when I first got it, and it used to smoke terribly on start up, but it doesn't do that now I've switched to mineral oil.


    In an engine with horizontal pistons, when the engine is stopped, the oil on the bores runs round the bores and collects at the bottom of the bore. On a K engine, if the bike is on the sidestand, it leams so that the oil cannot slide back down the bores to the sump, and gravity will cause the oil to collect against the piston rings. Synthetic oil seems to be very "slippery" and seems to be able to slide past the rings into the combustion chambers, causing smoke on start up. Mineral oil will also do this a little, but not so much. I found that even on the centre stand, unless I was parked across a gradient that favoured the return of oil to the sump, I still got smoke at start up.


    It could be that there is nothing wrong with the valve stem seals, and an easy way to test this is to park the bike on the centre stand, but park it in such a way that there is a definite lean causing the oil to run away from the cylinder heads (perhaps put a thin piece of wood under one of the feet of the centre stand?) This will ensure that the oil left in the bores can run down and return to the sump. If the engine still smokes on start up, then the seals are suspect, but if it doesn't then you can pretty much say that the seals are OK.

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