I have a 1969 B25, which I have just finished a nut and bolt restoration. When running it performs well, no flat spots, never misses a beat and accelerates well. I've just completed 350 miles and its last run was over 150 miles. I am running with a Boyer mk4 electronic system and a new Mk1 Concentric carb.
BUT from cold the only way it will start is zero throttle, flood carb, find TDS on the kickstart and kick. If you are lucky after the third or fourth kick it will start and when the rev's pick up you can open the throttle, otherwise opening the throttle will stop the engine.
When its warm , starting is in the lap of the gods, sometimes with throttle, sometimes without.