(Another excerpt from "the book". It's coming along...)
Several of us have talked about the non-centricity of the long threaded shaft that holds the spark advancer onto the 750's crankshaft: Honda even put out a bulletin about it. If you shine a timing light on your advancer and find the marks keep jumping back & forth (most often found on the 2-3 set), here's what it takes to set up and true out the shaft so this problem will go away. It smoothes the bike out, and often reduces plug fouling when you have a bike that only fouls the 2-3 or 1-4 plugs.
Below is a setup, using a dial indicator, to measure the position of this long, thin shaft. By the way, these often get bumped when the engine is taken apart, so you may find yourself fixing this more than once.
What you cannot see: the alternator cover is off of the other end of the engine, to facilitate turning the crankshaft slowly while you look for the low spot with the dial indicator. Use a plastic mallet or hammer to tap this shaft, at its base by the crank, until the runout is less than .001" (it's easily done with a little patience). Then, put the advancer back on right away, and add on a temporary points plate to protect it all.
