Bruce, you make a good point. My timing marks on the flywheel only show TDC correctly if the flywheel is in the right place. If the key is sheared the spark will still happen in the right place on the flywheel, even though the crankshaft is in the wrong place. I've just been lucky I haven't used my method on an engine that actually had a sheared key. It's OK for detecting the wrong breaker gap, at least.
With regard to those old Villiers engines with no flywheel key, when I didn't even know what the breaker gap was supposed to be it was a trial and error job for me. Took me a couple of hours to get a reasonable result on my old Atco. It would be really difficult to sell me one of those Villiers if I had a choice of a Briggs & Stratton.
