The Cessna 120-140 Association Forum

Before I run off and buy new Slick magnetos, I'll pose my situation here for ideas, feedback, etc...

I have had my 120, C-85-12, for 5 years. Every 20-30 engine hours, the left mag (Bendix SL-4N) timing retards from (28/30 I can't remember which) to almost TDC. Every dadgum time. It was sent back to Savage twice, but still does it. Savage said the mag tested great.

This mag is also hard to time, as we are at the limit of rotation with the studs at the appropriate timing setting. It doesn't seem to physically move off the stop, though.

So, what are your thoughts?
Thank you,
Silas

My guess is you have some left hand rotation parts in a right hand rotating mag, or it simply isn't correctly timed internally. Why the timing would shift without the mag moving is a bit of a puzzle. Usually it's due to either point wear or cam follower wear.

Bottom line, I don't think you've discovered everything that's going on.

Does it have an impulse coupling? You're not trying to time it with the impulse coupling in the picture, are you? (Do you know what "snap it out" means?)

It does have an impulse coupling, and we are not timing off that.

The first time we pulled the mags, the felt pad around the cam follower that pushes the point closed was worn quite badly. It was replaced during overhaul, but maybe something is still worn?

7645 wrote: ↑Sun Jan 27, 2019 5:12 pm It does have an impulse coupling, and we are not timing off that.

So, you turn it forward until it snaps, then back a little past TDC, then forward to the place where the points open, right?

That's correct.

Was the internal timing set by you or the overhauler? I also don’t understand why you are running out of adjustment with the slots? If that’s the case then move the magneto one tooth on the drive gear.
Just for the heck of it, I would suggest borrowing another Bendix magneto from someone and seeing if it falls out of time. That might narrow down the issue to being the Magneto that Savage said was in good condition.
JMHO, Steve

When you install the mag, do you have the engine at TDC or at 28 before? I assume you are pinning the mag at the e-gap position, right? You are installing it in the firing position, so you shouldn't have to move it much.

When we installed the mag, we had trouble shifting it one tooth for some reason, which is escaping me. So when it's installed with the marks in the appropriate spots, it's already close to the limit of rotation.

I'm inclined to let another A&P take a look at trying to reinstall and time the mag. Aircraft Magneto Services said they will look at the mag for the cost of shipping, so that'll be step 2.

Consider the problem may be with your procedure/instrumentation. What are you using for timing? Could it be the "flower pot" cone that clamps to the prop hub/spinner? Been around for years and I think it's made by Eastern. Has the weighted pointer on a center shaft, if so, has it been lubricated lately? If one gets in a hurry and doesn't double check the timing for repeatability, one may have the timing incorrectly set, therefore, later when checked again, it looks like the mag has shifted.
Years ago, I was doing an annual on my Bonanza and had problem getting repeatability checking timing - like to have drove me up the proverbial "wall".
Finally, lubricated the "flower pot" shaft which was "dry" and dragging causing the problem. If you have dual impulse, doesn't hurt to occasionally, with switch "off", etc. turn the prop and listen for the "clicks" of the impulse. However, WARNING, always turn the prop expecting it to fire!!

This happened to me many,many years ago - my start procedure was to pull prop thru 4 revs while listening for the carb sucking fuel, save starter and battery. One day it started on about the 2nd or 3rd pull, fotrunately, I was clear of the prop!! The old original mag switch had failed "on"!! Needless to say I replaced it with a Spruce ACS (Bendix knock-off at about 1/3 to1/2 the cost) with a field approval. Nothing new for old timers, but always treat the prop as if the mag switch in "ON". 2C and off the soap box!

Edd

Well, we went to retime the mag (using buzz box and Time-Rite) and we couldn't get the buzz box to indicate anything on the left mag. Further troubleshooting revealed that the cam simply wasn't pushing the points open. The points also appear to be adjusted at or very near the edge of the arm's travel, so there is little to no adjustment left. The screws were also tight, so it doesn't look like vibration shifted them.

This appears to be the same issue we first saw in 2014, before the mag was rebuilt.