Resistance for p-lead RPM pickoff

Does anyone else have a Dynon D120, (I)O-540, and a slick mag wired to get the RPM off the p-lead? I've tried a bunch of different values for the resistance in the line, and still can't get something that isn't erratic at some RPM range. The best I did was at 78Kohm it was steady up to around 2600 RPM, but above that would momentarily jump up to some ridiculous value like 3300. I tried going up in resistance from there, but then started getting erratic readings around idle in addition to the upper range. Fortunately, it seems to be good in the 1500-2600 range, so it's just an annoyance at full power.

I'm getting sick of soldering and shrink wrapping resistors, so hoping someone has a known good resistance value I can try. Next step would be for me to install a pot and play around with it in flight.

-Rob

For the archives, more info on this problem here:
http://www.vansairforce.com/community/showthread.php?t=71315

Eventual solution I used described here:
http://www.vansairforce.com/community/showthread.php?p=540591

-Rob

I helped my friends wire up their Dynon D120-equipped RV-10 (with Slick mags on the IO-540) and had to use 100K ohm resistors in series with mag connection from the back of the ignition switch to the Dynon D120 before the tach readings became stable and happy.

Based on that experience, when it came time to wire up the D120 in the RV-8 (IO-360, Slick mags) that's in my avatar, I simply started off with 100K ohm resistors and it's worked perfect since day one.

On both aircraft, I used shielded wire from the mags to the ignition switch (the usual ACS ignition switch model with L-R-BOTH-START) and then also used shielded wires from the back of the ignition switch over to the D120's DB25 connector. I put the resistors in series with the hot lead of the shielded wire very near to the ignition switch terminals. I wonder if having some length of shielded wire running "downstream" of the resistor, and before the D120, might help act as a small filter capacitor to smooth out any spikes on the leading and trailing edges of the waveform? If a "dirty waveform" is what's confusing the Dynon's tach inputs, I wonder if a small capacitor (e.g. 0.01mfd) from the tach input lead to ground, at the DB25 connector and downstream of the resistor, might help?