I've seen it before a few times, The rings start fluttering due to high rpm and piston ring groove wear, ( because of the constant high rpm) bit of oil gets in combustion chamber, usually only had a single hiccup, detonation, etc.
Piston crown gets hammered, ring grove closes on top ring cracking it in pieces Rings break up and start cutting cylinder , various bits of cast iron get into pin bore.
what red-line are you using? You need to lighten pistons if your going over 12K.
It would be a real good idea to get crank fully checked. Oh, check crankcase bearing bores, you will see fret marks but need to make sure bearing bores are still round. Any signs of piston touching head (pretty common if you have 'correct' clearance) If you don't have any marks on piston crowns or head squish band you probably have to much clearance and con-rod thinks it's a sine wave
Just went back and read all the posts. Pretty sure Cyorg was right about ring groove wear, possibly ring butting (not enough end gap) and resonance. Ring groove wear is something almost always overlooked on 'budget' re-builds where pistons are not being changed as they 'look OK' I've been told I'm OCD about measuring things but really I just don't like 'wasting' good stuff (change it cos your in there) or having engine blow up.
) I don't know how much information is available for them today but someone has probably posted tuning info from the 1980's? I found the reed blocks need a lot of work, you can make them flow much more than a Boysen set and still have 'street' like throttle response. Bigger carbs work, 34mm makes bike VERY peaky but 30 or 32 gives wider spread of power. (I think they have 26mm?) Oh, don't bother to work any of the intake covered by the reed blocks. Cut out piston bridge and match cutout to port width. Decent set of expansion chambers make a world of difference, even TZ350 ones will bolt on but they are bit noisy
