Hey Tim,
Not a direct answer, but consider using a hall effect tach sensor on your mag. It costs a little money and requires +5v to +12v regulated power, but you'll be able to do a mag check seamlessly.
I have a G3i with two mags and started using the +12v square wave signal from the electronic ignition for tach signal. RPMs got erratic at around 2500 and I ran into the same problem doing a mag check. My solution was to use a mechanical tach generator off the drive port on the back of the engine. That worked for awhile and was more stable, but broke in less than a year. The replacement was not accurate/stable. I should have gone with the hall effect sensor in the first place. No moving parts and it would have cost less than the two mechanical ones. I don't even want to talk about all the time and aggravation it cost.
-Russ