EI R1 tach reading low, jumping around

[DesertNomad]

I have a JPI 830 and an EI R1 tach. In cruise the JPI will read a steady 2330, while the R1 has started reading between 2310 and 2260 (so 20 to 70 rpm low). It will be 20 rpm low for 20 seconds or so, then fall to 50 rpm low for 10-40 seconds (guessing here) and then change again. Sometimes it just wanders around in the slightly low range.

At low power, they are within 10-20 rpm.

This happened once before about 2-3 years ago and EI recalibrated the R1. I am getting an optical tester to verify rpm this week, but I suspect the R1 is just not accurate. It gets data from the p-leads and the JPI uses some sort of magnet as I recall.

I see no indication that a mag is not always firing and the p-leads are missing some pulses. The JPI rpm is steady and the CHT/EGT are steady too.

Any thoughts?

 

[baboss]

Have you talked to EI about the isolators? They have internal isolators and your tach should have come with different value external isolators to use in the case of jumpy readings.