A new plug does not always work. Get a couple of the hottest heat range plugs with the smallest center electrode, these plugs are of a low resistance. This is the best way for the garage mechanic to go if a weak magneto is in question. A plug that is fouled or inoperable meaning the emf is inadequate to jump the gap of the plug rather it is going to ground due to a fouled path. Your Vincent at high revs, say 5,000 rpm will have a voltage far in excess needed to give a blue to white spark at points of emf leaks. Your Vincent will at idle with a good mag
jump a half inch gap with ease, spark plug removed H.T. lead to ground. Checking the spark plug for spark at atmospheric pressure does not insure spark in real life due to the much denser insulating air between electrode and ground due to the compression cycle of the engine. A Vincent in good nick will be able to jump a gap of 2 inches in the atmosphere running on one cylinder at around 3500 rpm. Last point, a new spark plug wire does not insure that it wont leak. Never take things for granted. Again, put it in a dark room and see what is going on.