The International Cessna^® 170 Association

Hey guys! I’ve been having a problem with one of my magnetos constantly coming out of timing. Here is the whole story:

I’ve had perfect magnetos drops for as long as I can remember until one day about a week ago I had my right mag go completely dead. Nothing, not even a little bit. Engine died on the ground right away unless I turned it back to the left mag or on both. So I had the following things done by a shop.

1. Pulled, cleaned, re-gapped and bench checked all of my plugs (champion). All checked out good. Even switched the top and bottom set to verify that this was not a plug issue. Problem persisted BUT now the engine only gets a 200-250RPM drop instead of a dead engine.
2. Isolated the mag switch with a box kit to verify that the switch was not faulty. Problem still persisted. (200-250 RPM drop)
3. Verify that both mags were grounded properly and the the p-leads were not grounding out. Substituted a spare set of leads and reattached to verify proper grounding.
4. Checked the leads with a high voltage testing unit. All leads checked out. Problem still persisted
5. Out on a BRAND NEW mag that had 30 minutes on a 172 in the mechanics shop and checked out good. Problem still persisted.
6. Now we get to the weird part…. As we were retiming the right mag we decided to check its timing again AFTER we ran it. To our surprise, the mag was consistently coming out of time significantly after the engine started. The mag was secured properly and did not move. The mag simply internally got out of timing. What could possible be causing this? Both the mechanics working on the airplane with me were stumped. Especially because the problem still happened even when we put on a new magneto and timed that. Has anyone else had this happen before?

My hunch is that the wiring harness on my right magneto is arcing on itself between the different leads. This is my suspicion because when the shop put on the brand new mag they used the same wiring harness off of my old mag. I’m curious if anyone knows of a place that I can buy JUST the wiring harness? Would this answer the question why we might have gotten some weird readings during our timing adjustments on the magneto? I’m stumped at this point outside of just getting a new harness!

Thank you guys in advance.

Chris H
N2675V
Jacksonville, FL

Make sure you are looking at the correct mag!

Before I spent $$$$$ on a harness, I'd switch mags left to right and see if the problem moves with the mag or not.

Did you check and reset the E gap when you had the mag out?

If you told me this mag was a new Slick mag, I'd tell you you probably got one of the Slick mags manufactured in an 8 month period last year that did not receive enough or any lube on the point block causing premature point block wear. And when I say premature I mean they lasted about 20 hrs. Slick has a service bulletin on this.

chrishartis wrote: ↑Wed Nov 13, 2024 12:22 am… one day about a week ago I had my right mag go completely dead. Nothing, not even a little bit. Engine died on the ground right away unless I turned it back to the left mag or on both. …

Sounds like a P-lead short. When the engine dies during a mag check (usually when accidentally turning the switch to off) it’s best to pull the mixture control, let the engine die, and then restart it. The problem with quickly turning the engine back on with the good mag is that the unburned fuel-air mixture in the muffler can ignite there, damaging the muffler.

chrishartis wrote: ↑Wed Nov 13, 2024 12:22 am5. Out on a BRAND NEW mag that had 30 minutes on a 172 in the mechanics shop and checked out good. Problem still persisted.

Are you saying that you had the same 200-250 rpm drop when checking the new replacement magneto?

Are these Bendix or Slick magnetos? Is the mag drop smooth or rough?

Here’s the Slick Service Bulletin Bruce was referring to.

When troubleshooting mags I like to remove the P lead from the suspect magneto and run the engine. This rules out problems with the switch or P lead, and also confirms that the leads are going to the correct mag.
Did you reuse the old impulse coupling when you tried the new mag?
The harness has no effect on internal timing. Sounds like you could be missing something simple.