magneto timing

What engine are you referring to that would use an impulse-start magneto, but won't tolerate at least 20 degrees of spark advance when running?

As soon as the engine "catches" and begins to rev up, at a few hundred RPM's, the impulse will drop out because of centrifugal force acting on the weights. When the impulse drops out, the timing instantly advances, all at once, to full running advance.

The number of degrees of advance that the mag advances is determined by a spec called the "LAG ANGLE" of the impulse.

There are different weights available for common mags, as was originally spec'd by the engine manufacturer, and usually the "lag angle" is marked on the weights.

Therefore, if, for example, you have a magneto set up with a 12 degree lag angle impulse, the timing advance at startup, measured in crankshaft degrees, will be 24 degrees, since the crankshaft turns twice as fast as the magneto, and rotates 2 degrees for every degree that the magneto rotates.

Therefore, when checking the running timing with a timing light, you should see the initial timing, (close to 0 degrees from TDC) plus the timing advance of the impulse, which will typically range from 20 to 30 degrees.

I hope I've written this clearly enough to make sense!

To sum up what I said before, the initial timing near TDC is important for safe starting, and, if you don't like the running timing, change the impulse weights to a lag angle that meets your target for running timing.