Pull of the vac tube at the carb and suck (not joking) to check the advance is working. The diaphram may be faulty.Also check for wear in the dizzy bush by trying to move the rotor back and forth. If its worn no ammount of setting the point gap correctly will help. A condenser may be the culprit also.
Somewhere else to check is wear at the carb spindle. Wear here makes it very difficult to tune correctly.
Lastly, if you have regular chuf chuf noises at the exhaust then a burnt valve (or two) may be the problem.
