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M9F Magneto

Posted: Sun Jul 22, 2018 1:41 pm
by SHIPCHIEF
Last week I experienced a magneto failure during run-up...for departure to the Corvalis Corkcrew IAC contest. :cry:
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...

Re: M9F Magneto

Posted: Sun Jul 22, 2018 4:07 pm
by SHIPCHIEF
Here is a pic of the mag with the coil and coil extension piece in 'exploded' layout

Re: M9F Magneto

Posted: Sun Jul 22, 2018 4:12 pm
by 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.

Re: M9F Magneto

Posted: Sun Jul 22, 2018 6:24 pm
by 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.

Re: M9F Magneto

Posted: Sun Jul 22, 2018 6:27 pm
by SHIPCHIEF
The missing pic: