Re: magneto timing

Posted by Bob on October 27, 2004 at 09:07:16 from (66.163.134.177):

In Reply to: magneto timing posted by mike941 on October 27, 2004 at 08:26:13:

A magneto with a working impulse coupling is typically timed so the impulse snaps and the spark occurs at TDC or no more than a very few degrees before TDC. If timed earlier than that, kickback will occur at starting, which will damage the starter (human or otherwise).

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!

Follow Ups:

• Re: magneto timing mike 941 18:52:42 10/27/04 (1)
  • Re: magneto timing Bob 19:38:06 10/27/04 (0)

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