M9F Magneto

[SHIPCHIEF]

Last week I experienced a magneto failure during run-up...for departure to the Corvalis Corkcrew IAC contest.
I flew Sportsman (poorly) in my RV-8.
This weekend I worked on the Yak. Of course, I started by accidentally selecting the wrong mag to disassemble.
I figured it must have been the one without the 'Shower of Sparks' because the engine starts and runs fine until #2 mag is selected for the mag check, whereupon it quits.
Well...#1 mag is on the Starboard side. It got inspected & points gap cleaned & confirmed, the cam block cleaned and lubricated and the coil and P-lead circuit all ohm checked. Also the Mag selector switch on the panel was removed, unwired and checked with an ohm meter for continuity and proper operation. That's when I discovered the #2 mag is on the Port side!
The problem is that the #2 mag High Tension Coil output post shorted to ground. The distributor cap is a bit burned on the edge where it isolates the post. The soft insulating tube around the post failed it's mission, and the heat from the arc left it's mark.
The reason that this mag still starts the engine is because the start coil doesn't use the Mag's coil as a Bendix or Slick does. The M9F uses a separate start coil that powers the spark plugs directly thru a separate circuit in the distributor cap and a separate rotor finger.
I found a little support for these mags on the internet:
https://magnetoeblog.blogspot.com/
I removed the offending mag and took off the coil and separate post. After cleaning off the coil, it appears to check out OK with the ohmmeter.
I'll post some pictures next.
Meanwhile, I just found a YouTube video of an M-14P BENT CONNECTING ROD internal inspection...

[SHIPCHIEF]

Here is a pic of the mag with the coil and coil extension piece in 'exploded' layout

[SHIPCHIEF]

You can see the damaged insulating sleeve on the extension piece in the previous pic
This is how it goes together. The tab at the base of the coil is the contact for the P lead. Grounding this terminal shuts off the magneto.

[SHIPCHIEF]

Here is a pic of the coil firing through a piece of 8mm MSD brand spark plug wire that replaced the original. The spark easily jumps the 10 mm air gap when the drive end is turned by hand.
The long piece of wire from the coil (in a previous pic) attached to the points, which were dirty. Once cleaned and showing 0 Ohms when closed, the magneto Sparks again!
Next, clean & lube the points cam, stake the tab washers and make the end connector for the new coil extension piece.

[SHIPCHIEF]

The missing pic:

[tarabusi]

Hi, i'm having the same problem on my Yak 52 here in Italy. When i set SW position to 2 the engine quits.
From what i see hee the culprit should be the coil.
Do you thing i can remove the coil without removing the whole magneto?
And once you have removed the mag, did you have to re-time it? i have no tools to do this.

Thanks a lot
Attilio
LY-ASQ

[SHIPCHIEF]

Attilio,
Sorry I didn't return to this site for a while, I hope this helps someone...
You will need to remove the mag. The coil might not be bad. The coil output might be misdirected to ground as mine was. This is not an uncommon occurrence.
The clamp bolts that hold the magneto in place and hold it in rotational position to set the timing are hard to get tools on. a variety of tools may be required to remove/replace them.
If you do not rotate the propeller, and note the position of the magneto distributor rotor, make sure the rotor aligns to the same spot upon reassembly.
The engine and the magneto have an unusual gear ratio, so I have assembled it out of phase. It took some 'head scratching' to figure it out, but you can do it.

Good luck,
Scott