RPM spikes

[Transregional]

I have an EMS D10 installed and all is working fine except for the Tacho. I have two inputs, one from a Mag and the other from the Lightspeed CDI system. Individually they each read the correct RPM and together the reading is steady and correct. During run-up, both systems read correctly. RPM red line is set at 2800. During take-off the RPM constantly has over–read spikes beyond 2800 causing the alarm to be triggered multiply times until such time as the RPM is reduced to 2400, at which point the reading is steady once again. Also during take-off the entire EMS will shut down and restart again, whereupon the unstable RPM spikes and alarms will continue until RPM reduction. Happens every take-off. Any help would be appreciated. Take-off actual RPM is not above 2700 as confirmed by previous tacho, now not used.

 

[Dynon]

So the rpm spikes are likely coming from the magneto side. But you should check this by doing a higher throttle run on just the mag and on just the lightspeed to see if you can replicate it without doing a full takeoff roll on only half your ignition system. In other words, the EMS is only watching one of the two nominally, but you don't know what it is. It's likely to be the one that's on the ignition switch position that's next to "both".

If you confirm it's from the magneto side, you can increase the series resistance on the p-lead some. Go up in 30k increments (you should have a 30k resistor on there already).

As for the shutdowns - do you see it ONLY on the takeoff roll when you're truly at full throttle? Or also in flight too? What about when you're on initial climbout and you're still at max(ish) rpm?

 

[Transregional]

Thanks for that.
Regarding the RPM spikes, I already have a 30K resistor on the mag and turning the mag off (I don’t have the standard key-type ignition switch but rather two separate toggle switches –one for each system) makes no difference to the spiking RPM so it looks like the Lightspeed ignition is the cause. I thought the tacho output from that was supposed to be 10v peak but I’ll try a 30k resistor in that line and see what happens. Also, if I keep cancelling the warnings, they eventually do not re-occur again after reaching an altitude of about 4500ft and above at max RPM and MP.
The EMS shuts down and restarts once only and always during the take-off roll at full power. After that it is all ok
Thanks for your help.