M20J Ignition/Starting Troubleshooting

[Joe Linnebur]

Good evening everyone;

I've looked over the previous posts regarding starting issues and I figured It'd be easier to just start a new thread with the issues we're having:

Aircraft: 1979 M20J

Engine: A3B6D - with a Bendix D3000 Dual mag (overhauled in April, and thats when these issues began)

Impulse Coupling...it's not Shower of Sparks

We overhauled the D3000 Dual mag in April because of a bad running mag. Overhaul worked out fine. After the overhaul, we noticed that the previous starting technique wasn't working so well. On the first attempt, the engine would only turn over, with no interest in even attempting to act like ignition was occurring. After a few more attempts, it usually would reluctantly start. Mag checks during the runup were always well within limits (less than 100 rpm drop, smooth running, etc) and never indicated any issue with either mag...it was just starting it was the issue. 

I've noticed that some starts occurred after running the starter and after 10 blades or so, bringing the key back to "both", only to have the engine actually come to life. This has happened on a number of my starts. There have been other starts where it turns over 10 blades, key off, back to "start" and then push and it starts after 2 blades. Up to now, I've been the only one to be able to start it, and there seems to be no consistency in any technique, so lets not go down that rabbit hole. We've tried multiple new techniques and there's no consistency at all. We originally though it was a flooding issue, but that's been determined that it was flooding due to the multiple start attempts. 

 We recently had the mag pulled and re-inspected. No change. Its now with our main A&P. We discussed checking the ignition switch and if its grounding out the entire mag while in the start position. Based on my experience where it wouldn't fire until I selected "BOTH" seemed to be a possibility. He thought he may have found it intermittently grounding out the mag and "fixed" it, but I'm not sure "how" he did (I didn't have the conversation with him). Fast forward a day (today) and a partner was attempting to bring it home. Neither he or the mechanic could get it started. This was after he pulled the mag off and did another interior check of the components. 

I have noticed that when I run the prop through, looking at the 25 degree and TDC marking on the starter ring gear assembly, the impulse coupling tends to actuate well past the TDC position (about 3-4 teeth on the gear, plus or minus a couple. math tells me about 2-3 degrees past TDC). I'm not sure if our impulse coupler is actuating too late in the piston movement process thus making it a bear to start. #atmywitsendhere 

Have any of you all run into a scenario such as this where it was obviously ignition related, yet completely baffled you as well as your mechanic? Ours is baffled at this point. I'm about to recommend replacing the ignition switch based on previous posts as it seems like the mag is grounding out when in the "start" position intermittently. Any feedback on past experiences would be greatly appreciated to push us in the right direction. Thanks everyone

 

 

[KSMooniac]

Who overhauled the mag?

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[N201MKTurbo]

What are you timing it to? 25 deg BTDC? 
 

Are you sure it isn’t grounding either mag in the start position? Put the timing test P leads in and try to start it. That will make both mags hot.

[Joe Linnebur]

9 hours ago, KSMooniac said:

Who overhauled the mag?

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It was a local facility. It's been removed and reinspected by our current IA (who overhauls these dual mags a lot) and he found everything put together correctly. We're thinking internally its looking solid. Once it starts, its 100% fine...its just getting it started.

[Joe Linnebur]

2 hours ago, N201MKTurbo said:

What are you timing it to? 25 deg BTDC? 
 

Are you sure it isn’t grounding either mag in the start position? Put the timing test P leads in and try to start it. That will make both mags hot.

Timed to 25 BTDC. That's kind of what I'm thinking right now is ignition switch issues. One idea for troubleshooting was starting it with the P-leads removed to see if that results in any difference. 

[EricJ]

Just now, Joe Linnebur said:

Timed to 25 BTDC. That's kind of what I'm thinking right now is ignition switch issues. One idea for troubleshooting was starting it with the P-leads removed to see if that results in any difference. 

Was just gonna +1 to try it with the p-leads disconnected.   It needs to have the timing plugs in place to assure that both sides are hot.   That should help isolate whether it is a mag or a switch issue.

It may also be worth checking to make sure the switch is configured properly so that both sides are hot during start.   That's one of the advantages of the dual mag.

[KSMooniac]

Were the capacitors replaced?

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[Joe Linnebur]

27 minutes ago, KSMooniac said:

Were the capacitors replaced?

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Yes, both capacitors, points, coils, etc. 

[Joe Linnebur]

1 hour ago, EricJ said:

Was just gonna +1 to try it with the p-leads disconnected.   It needs to have the timing plugs in place to assure that both sides are hot.   That should help isolate whether it is a mag or a switch issue.

It may also be worth checking to make sure the switch is configured properly so that both sides are hot during start.   That's one of the advantages of the dual mag.

Good info. I’ll pass this on to our A&P. Appreciate the insight

[Pinecone]

When it starts when you release the key, it sounds like the switch is grounding one mag when in the Start position, and it is grounding the wrong one (the one without the impulse coupling).

[EricJ]

2 hours ago, Pinecone said:

When it starts when you release the key, it sounds like the switch is grounding one mag when in the Start position, and it is grounding the wrong one (the one without the impulse coupling).

Dual mag, the impulse coupling drives both sides.

[Pinecone]

OK.  Scratch that idea.

Then check that the switch is not grounding BOTH mags in the Start position.

 

[A64Pilot]

Try cleaning and gapping the plugs, only time I had real issues starting my old 540 with a D mag that’s all it was.

[Joe Linnebur]

Just a follow-up to this for future answer seekers:

We ended up replacing the impulse coupling. The spring tension was not adequate. Starting is back to norm. Thanks all for the ideas and experiences