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Mag timing questions

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00Dan

Well Known Member
Checking my timing and formed a couple questions.

First, is there a tolerance for the mag sync? Obviously I’m aiming to get them identical, but for say just checking, what’s “close enough” to not bother adjusting?

Second, my impulse coupling seems to be snapping late when I pull the prop through by hand. It’s a 25* lag and I have the mag set at 20* BTDC. When I pull the prop through to release it before checking the timing, it’s snapping at ~10* ATDC rather than the 5* ATDC I’d expect. Is this just due to the slow speed hand pull or is the coupling suspect?
 
00Dan said:
Checking my timing and formed a couple questions.

First, is there a tolerance for the mag sync? Obviously I’m aiming to get them identical, but for say just checking, what’s “close enough” to not bother adjusting?
Click to expand...
Dunno. Whats the book say? As a matter of pride, I like to be spot on to a TDC reference (that is likely +-1 or 2 degrees off).
00Dan said:
Second, my impulse coupling seems to be snapping late when I pull the prop through by hand. It’s a 25* lag and I have the mag set at 20* BTDC. When I pull the prop through to release it before checking the timing, it’s snapping at ~10* ATDC rather than the 5* ATDC I’d expect. Is this just due to the slow speed hand pull or is the coupling suspect?
Click to expand...
The impulse lag is pretty much baked in by the geometry of parts within the impulse assembly. The stated lag should be evident with a slow pull through. l doubt that the lag is ever specified by dynamic measurement since very slow cranking speed is where the exact lag would be more important.
Two possibilities come to mind: wrong impulse assembly. Maybe a 30 degree assembly. Or worn.

I would want to know. If an impulse comes apart from being worn, it could wreck the engine and put the aircraft on the ground.
If it is a wrong part, but starts OK, I would live with it.
 
You need to post exact engine model to be certain. Most of the four cylinder Lycomings are timed at 25 degrees except for some of the angle valve are timed at 20 degrees. 22 degrees will definitely help with high CHT. Properly maintained and inspected Bendix mags are not going to fail if the impulse coupling is properly inspected.
 
This is a slick mag with a tad over 200 hours on it, on an O-320 with 10:1 pistons. I am intentionally running 20* BTDC timing for detonation margin. The mag is dataplated to have a 25* lag angle on the impulse coupling.
 
As far as tolerance between the two, most certified airplane POH's call for a max differential between the two mags of 50 rpm during the preflight mag check. That's where a timing split between the two would show up.
 
300 hours on 0 320 with 10-1 pistons. 22 degrees for controlling CHT until rings were seated. Now 25 degrees. No issues with engine. I have a cowl flap and plenum.Engine runs a bit lean at full throttle. I just haven't gotten around to drilling the jet.
 
Pulled the impulse mag today and inspected the coupling - nothing appeared broken and there was no abnormal wear on either paw. I went ahead and pinned the mag to reinstall it and per the buzz box it fires exactly where I want it to normally.

However, if I back the prop up to reset the impulse coupling, then slowly bump the prop through, the coupling isn‘t releasing until 13* ATDC per my digital level I attached to the crush plate. I do feel resistance starting at approximately TDC commensurate with the paw catching and winding up. I’m now at a bit of a loss as to whether I should try it or pull the mag assuming something is wrong with the coupling.
 
00Dan said:
Pulled the impulse mag today and inspected the coupling - nothing appeared broken and there was no abnormal wear on either paw. I went ahead and pinned the mag to reinstall it and per the buzz box it fires exactly where I want it to normally.

However, if I back the prop up to reset the impulse coupling, then slowly bump the prop through, the coupling isn‘t releasing until 13* ATDC per my digital level I attached to the crush plate. I do feel resistance starting at approximately TDC commensurate with the paw catching and winding up. I’m now at a bit of a loss as to whether I should try it or pull the mag assuming something is wrong with the coupling.
Click to expand...
A piece of amplifying data - I checked the base timing of the mag before I pulled it to inspect the coupling and found the timing had not drifted any from when it was first set. Only the snap point of the impulse coupling appears to have drifted.
 
I wanted to post a follow up on this as a lesson learned for me, and hopefully for others. I pulled the mag and sent it off to Aircraft Magneto Service in Stevensville, MT. Steve, their slick tech, theorized over the phone that the internal timing had drifted and it probably needed a set of points after I described my observations to him.

Once he got it in, he called to let me know the internal timing had drifted ~10*, consistent with what I measured. The more concerning things were the various slick service bulletins that applied, SB1-15A and 2-19A. Both of these had been out for a bit now but surely there may still be affected mags floating around in the wild, since at least for me, accessory mfg SBs don’t come across my feed often. In particular to my case, the 2-19A inspection revealed a rivet on the impulse cam that was beginning to work loose. This could have resulted in an unscheduled disassembly of the impulse coupling inside my accessory case, which is definitely not ideal.

Lessons learned - listen to your accessories when they start talking to you, and second, take the time to run the serial numbers and see if there are bulletins or other applicable data from the manufacturer - don’t get caught unaware.
 
That’s a really nice report.

Those guys at Aircraft Magneto Service are the best!

They told me once that an impulse coupling is the greatest source of failure in a magneto. Something to think about.
 
I’m confused how you are setting and checking timing.. you should be approaching #1 TDC, continue pulling until the mag impulse coupling clicks, then you can back up and go forward, setting timing. You don’t have to back up and reset the impulse coupling.
 
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