When you rotate the engine slowly by hand, the magneto impulse will trip at TDC.
If it is set to trip any great amount earlier than TDC, there is a risk of kickback while cranking.As soon as the engine "catches" and speeds up to over a couple hundred RPM's, the impulse no longer "catches", and the timing jumps to full advanced running timing. The actual amount of advance is determined by the IMPULSE LAG ANGLE, usually marked on the impulse, and selected by the tractor manufacturer, and it does vary. The actual advance is the impulse lag angle doubled, as the mag turns 1/2 crankshaft speed. A typical lag angle might be 14 degrees, which would give you a running timing advance of 28 degrees. That is why the timing mark intended for "static" timing, by slowly rotating the engine until the mag "snaps" is not lined up when viewing it with the timing light... it is then at full-advanced running timing. I don't know what the impulse lag angle spec is for a CUB, offhand, but the Fairbanks mag (as far as I know) is NOT original equipment. If you know the flywheel diameter, you can determine the circumference, and divide that by 360 degreee, which will tell you how far the running spark is actually advanced for each inch it's off the static TDC mark.
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