Yb100 Spark plug problems

Hi

Hope someone can help me with some ideas.

I have a 1989 YB100 purchased from ebay and lovingly restored over the last 2 years including new bearings oil seals etc. It has been running reasonably well until about a week ago, when it started 'breaking' spark plugs - a new plug would run for only a few miles and die - ie weak or now spark. The colour of the plug was dark grey/black but not oily suggesting a rich mixture. I adjusted the carb and oil pump to try and acheive a leaner mixture - but no success, and today on a ride out it went through 3 plugs and finally died completely and had to be trailered home!

An initial strip down indicated damage to the piston by a plastic substance which could only be the induction disc valve. Sighting through the induction port seems to comfirm this - so I have a job to replace this as well as the piston, with possibly a rebore as well! There may be another cause of the disc valve damage which I may find on further dismantling, but has anyone heard of this happening before - and was the 'plug destruction' a symptom of the impending failure?

I am interested in any thoughts on this and how to improve the mixture settings on this bike for th future.

Thanks

Geoff