The slide needle/nozzle in those carbs is also a wearing part. I had an old 70's "barn fresh" xr 80 and the needle had vibrated in the nozzle for sooooo long(decades of changing water on the back 40 and barn yard hot laps) that the needle/nozzle were worn thin allowing much more gas than the motor could use. Replaced the needle and nozzle and it would cleanly rev out again. Another snag with that bike was that the edge of the exhaust valve seat had been eroded and wouldn't allow a good seal resulting in low compression allowing it to fall on its face, when the throttle was opened too quickly.
Great thread, which has me eyeing the classifieds for that canary yellow cl66 type, I passed up so long ago... Might have to pick up that CL175, that keeps turning up on the local craigslist, as a tide me over!