I tried everything. Cleaned and rejetted carb, checked valve clearances, checked timing. Rejetted carb.To put it shortly, I became a pro at removing the carb.
After lots of frustration, swearing and thinking of selling it, a friend suggested that I must cut all of the cable-ties off that were keeping the throttle cables neat and in place.
Problem solved. Sometimes the simplest thing makes the most difference.

