Erratic RPM with Pmag after 250 hours

[Tuiliere]

Hello,
I have an RV7 with an IO375 from Aerosport power, 2 Pmags and 2 Skyview 1000 touch. my plane has been flying for 6 years and about 250 hours. for a few hours I have noticed erratic RPM without finding out where it can be from. In flight I have tested both Pmags separately but the problem persists with one or the other of the selected pmags. so it is not a Pmag that is causing the problem. Could it be the resistor ? Broken ? the rev counter numbers work fine for a few seconds then jump forward 100 or 150 revs or 200. it's never the same. Does someone have an idea ? In advance thank you.

here is a video of my problem :

 

[CanardMulti]

I have Pmags and that looks to me exactly like my rpm did when the ground wire in the terminal block on the mag loosened. Apparently the Pmag's ground was intermittently only thru case contact. I hit the wire end w/ a little contact cleaner, reinserted it in the block, and snugged the screw securely. RPM now rock solid.

 

[jakej]

And…. you should remove the resistors, they are not used or needed with the p- mags.

 

[Tuiliere]

Thank you very much for your reply. I am going to test your solution.

 

[N941WR]

The P-mag ground, pin #1 on the P-mag's green connector, should go to independent case bolts, not a forest of tab ground block! Two P-mags, two different case bolts.

I cannot stress this enough!

If you put it on a forest of tabs, the tach signal, which is analog, can pick up noise from other equipment using the common ground block.