engine rpm 1/2 on left mag? anyone else had this problem?

I use a RMI for engine monitoring, Specifics,,,, on the mag check on right mag everything is normal. On left mag rpm indicates 1/2 but the engine doesn't feel like a loss of power of this magnitude. So I asumed this was a indicating problem. When flying the RPM is erratic, on right mag it reads 2300 rpm and you get a mag drop with a rise in egts on all cyls. If you go to the left mag RPM is half with a rise in egt on all cyls. The mags have about 250hrs since ovhl and they are slick mags. I think they are 4070series.

RMI Tach issue

I had this same problem with my RMI engine monitor. It happened several years after installation and had been working great. I thought sure I had mag problems and worked on them and the coupling capacitors on the mag switch but it didin't help. I took it to the lab and hooked up a pluse generator and scope and found the coupling resistor in the RMI was too high a value. I don't remember what resistor it was or the value that I replaced it with. Call RMI. I think they are aware of the problem.

Ken