I had an inquiry this week about how to time the rotor on a J4 magneto and since there is a spare magneto in one of my parts drawers, I took a couple pictures and emailed to the member making the inquiry. I thought maybe they would be beneficial to someone else also. To make it easier to see, I used some white paint on the mark on the rotor.
First picture is how the rotor and pinion gear should be timed. This is also how it should look just after the magneto trips at TDC on #1 cylinder. It is easier to get the gears right, if the magneto is off the tractor, then you can turn it backwards to get the marks lined up rather than fighting the impulse.
this second picture is as the gears look as the engine approaches TDC of #1 cylinder just before the impulse trips.
this is the position of the governor to magneto drive slot with the engine at TDC of any cylinder.