The symptoms are similar to those of pg42, restored LD 150 AVV, runs for 3 miles, then cuts out. After around 40 minutes, it struggles to start and is then ok for another few miles. When it cuts out, the cylinder head and plug are very hot, but there is still a spark, fuel runs ok, carb has been stripped and cleaned. Coil and condenser are original, timing is 26 BTDC, plug is B6HS set correctly, points are new and set correctly. The coil tests ok for resistance when cold. Auto advance mechanism is OK, as judged by the timing strobe test and fuel filler cap hole is clear. Plug lead is new as is suppressor cap.
I have since taken the scooter out with another plug and the same thing occurred. I then changed the plug again and it didn't restart so I can discount the plug as a problem. This time, instead of cutting out entirely, as the engine fades, I could keep it spluttering for a few seconds by pumping the throttle, and then it dies completely. The Ht coil is hot, you can touch it, but it is hot. Depending on any responses, I may try substituting the condenser?