I hve the EI R1 tach and have had intermitten problems on the right mag RPM only (during run-up)
After a year of troubleshooting, I have isolated the problem to the new Slick Mag installed a year ago
EI is a great company with great Products and fantastic tech support
The EI tach gets its reading from voltage spikes taken on the P lead and pads them with resistors
Champion Rep. told me they don't recomend this type hook up for safety reasons, due to an instrument failure causing a ground in the system and killing the engine. They recomended useing a magnetic pickup in the back of the mag @ vent plug
Champion Insists that the Mags are fine because the engine is running great and they don't control the voltage being supplied to the P lead.
Ei does not make a magnetic pickup that will work for this tach. and gave me some more resistors to wire in but they didn't fix the intermitten problem
I love the EI tach, size and looks, and don,t want to change it out to another brand. My A&P wants to change out the condenser in the right Mag ( 4 HR LABOR) I don't really want to spend any more money on this intermitten problem. They are only 500 HR Mags and I now have 150 hours on them.
I don't really like the fact that a resistor, they call it an isolator, Is suppposed to keep the engine running in the event of a failure.
Good luck with your decision. Ty