Hello Grumpy,
Well, I took the machine outside this afternoon, and performed that Oil-Cap-removal test you requested several times. The result each time was that the motor didn't miss a beat at any stage, nor huff and puff or chug in any way shape or form. I imagine that you will interpret that outcome as meaning that there is probably a pretty good seal being formed in the bore by the piston rings...
Just by the by, I was a little surprised by just how many small oil droplets were getting flicked up high enough to make it out of the top of the Oil Filler Tube when the motor was running with the Cap off. There was truly a steady supply of them!
I can see now from looking at the Exploded Parts Diagram for this motor that it does indeed have a dedicated Oil Slinger, but I was still surprised by the amount of spray because I was hitherto under the mistaken impression that only the I/C variants ran Slingers. In light of this, and given that it appears that the Rings and Bore are probably in good order after all, I'm swinging back to thinking that the problem could be due to liquid-form oil, after all. Or in other words - a case of poor sealing from the O-Ring (albeit still in perfect condition) at the base of the Filler Tube, as well as from the Seal under the Cap...
Mind you, that still wouldn't explain why the Plug has been running dark since the thing was new - The "oil vapour" theory works better with that issue...
Any further thoughts would be most welcome!
Best Wishes for now,
Mick.
