This is for magnetos with hex drive in order to position them correctly, so you can get the engine timing right. It can be a real pain in the ass to get the mag in the right spot between the front exhaust pushrod and the engine case and what not, on Knuckleheads, Panheads and Shovelheads.
It seems easier to read and understand, then to do. (at least it was for me. I took one of them special little buses to school.) It’s like one step forward, two steps back, do the hookie-pokey turn yourself around and drop the mag in place. Best of luck to you. :?)
1 tooth on gear =72 degrees
1 hex flat = 120 degrees
1 rotation of magneto 720 degrees (1/2 engine speed)
1 hex forward, 1 tooth back (120 – 72 = 47) degrees advance
1 hex back and 2 teeth forward (72+72-120 = 24) degrees advance
What does a magneto look like on an oscilloscope?
Some pictures of the scope with the bike running. Magnetos are AC voltage and look quite different from a regular ignition. Magnetos rely on current build up from the rotor magnets spinning in the laminates built into the housing of the mag. Current produces voltage in the primary of the coil. and then the points open and the collapsing field produces high voltage in the secondary part of the coil for spark to occur.
I’ll get back to this with scientific explanation and proper terms. In the mean time here are some full color squiggly lines to look at of a mag that works and starts easily.
YELLOW is Front cylinder
BLUE is the current probe, both cylinders
PINK is the rear cylinder
GREEN is both cylinders
This mag has one of the Morris Magneto’s single fire wire sets on it, which is why you see front and rear and both. You can see how the current builds up and the the points open and you get a spark line.
Below is the same bike, but at a fast rpm, like about 2000 rpm. The green line is a proper ignition probe and the yellow and pink I just used a piece of wire wrapped around the spark plug wire and hooked the oscilloscope probe to the wire to pick up the signal.