RPM jumps at high power

[Sputnik]

I'm running a LYcoming O-320D2B (Bendix mags) and using the magneto spark-counting feature to determine engine RPM. It works fine on the ground, but as I fly and increase speed to get RPM near 2700, the RPM counter starts intermittently jumping to 3300 and back.

I have tried, endlessly, adjusting the sensitivity rheostats. Unfortunately, there's no happy medium; it either won't read at all, or at a slightly higher value it will read this spurious value at high power.
Is there a simple band-pass filter I can insert into the circuit to clean up this input and get a nice power reading? Has anyone solved this issue?

Thanks.

[sparxfly]

I have struck a couple of installs with this problem- some you can fix by tweaking the sensitivity, but I have found the quickest, simplest, most reiable solution is to fit a UMA rotation sensor to the mag body. Aircraft Spruce sell them (10-02432, 10-02433). Rock solid across the rev range.

[Sputnik]

Neat, but I don't think that can help me. That unit is 4.5" long, and I don't have that much available between the firewall and the magneto.

[sparxfly]

Don't believe that 4.5" long dimension.
The sensor fits in an inspection hole in the side of the magneto body, and will stick out about 1" max including the wiring bend.

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