John Schwartz told me a story one time of a Guzzi he just couldn't get running right...with a timing related problem. He kept setting the timing, but it would get off again after running for a bit. Nearly drove him crazy. Turns out that the spring pin that held the gear on the bottom of the distributor shaft had sheared! Certainly not a common occurence, but something to keep in your back pocket of things to check when all else fails.