Magnetos

[SessionDrummer]

I bought a 1977 M20C last year, and I'm a first-time owner.

Just did the annual. Prior to that, the mags had been running rough during run-up and were due for a 500-hour. We sent them out, and no overhaul was necessary. But, they got a good once-over. My A&P was comfortable with the result, and let the plane out of maintenance. But, by my second flight, the run-up was rough again. Sent her back to maintenance and replaced the ignition leads. Everything checked out with the mechanic, and they ran smooth on my first couple flights. But, sure enough, they're running rough again.

The mags are Bendix, and the engine is the 0-360-A1D. I'm starting to wonder if it's something else.

Any ideas here?

[MikeOH]

What's your leaning procedure?

[Ragsf15e]

24 minutes ago, SessionDrummer said:

I bought a 1977 M20C last year, and I'm a first-time owner.

Just did the annual. Prior to that, the mags had been running rough during run-up and were due for a 500-hour. We sent them out, and no overhaul was necessary. But, they got a good once-over. My A&P was comfortable with the result, and let the plane out of maintenance. But, by my second flight, the run-up was rough again. Sent her back to maintenance and replaced the ignition leads. Everything checked out with the mechanic, and they ran smooth on my first couple flights. But, sure enough, they're running rough again.

The mags are Bendix, and the engine is the 0-360-A1D. I'm starting to wonder if it's something else.

Any ideas here?

So to elaborate on @MikeOH question… what do you do with the mixture immediately after starting the engine? Describe it exactly…

[SessionDrummer]

Hi Mike. Thanks for chiming in.

Funny you should ask. My A&P suggested that I prime less before starting, but that didn't help.

To answer your question specifically: Starting in hot weather, I go full rich and prime twice before ignition. In flight, I lean for 10-11 gph (usually square at 24/24). I sometimes start the leaning process during climb since, in once instance, I noticed engine roughness during climb-out that resolved itself when I started leaning. 

My previous time is all Cessna, Piper, and Diamond. Trying to learn the quirks of this aircraft and engine.

[SessionDrummer]

Oh, and I'm seeing your other question. Immediately after starting, I usually lean it out for taxi and then go full rich at run-up.

[EricJ]

Do you have an engine monitor?

 

[SessionDrummer]

No engine monitor. There's a fuel-flow indicator and an old-school cylinder head temp gauge.

[Ragsf15e]

23 minutes ago, SessionDrummer said:

No engine monitor. There's a fuel-flow indicator and an old-school cylinder head temp gauge.

So with an engine monitor it would be easier to isolate the issue, so our leaning questions are a stab in the dark, however, how do you lean for taxi?

Most of us will start the engine, set 1000-1200 rpm, check oil pressure, then pull the mixture back until the engine starts to stumble.  Like actually sputter a little, then twist it in enough to just barely make it smooth.  If you add power to taxi, the engine will sputter and maybe die if you don’t add a slight bit more, then take it back out when you go back to idle.  For runup there are various techniques, but somewhat more rich is fine, no reason for full rich though.  Below 65% power you can set mixture anywhere and not hurt anything.    On the ground all you get from rich mixture is fouled plugs and potentially some cht cooling if you have to sit in very hot temps for a long time.

Im not saying you are fouling plugs, but it’s one of several possibilities.  Leaning very aggressively on the ground usually stops that issue completely.  
In the air, do what you were already doing.

[MikeOH]

Rags beat me to it.  You need to lean VERY aggressively on the ground, just as he described.  It needs to stumble if you advance the throttle much beyond 1200 rpm.

As a reference for cruise, at 8000 I'm WOT, 2500 rpm, and leaned to 8.5-9.0 gph.

The description of how the problem goes away after maintenance, and then returns is what suggests plug fouling, but I'm not smart enough to claim something else couldn't be going on, however.

[SessionDrummer]

Thanks sincerely for this info. I will apply what I'm learning here and see if there's improvement....

[47U]

2 hours ago, SessionDrummer said:

Just did the annual. Prior to that, the mags had been running rough during run-up and were due for a 500-hour. We sent them out, and no overhaul was necessary. But, they got a good once-over.

Did the shop you sent the mags to swap out any parts?  Condensers or coils?  How long had it been since the mags had mx (or full overhaul) prior to your recent annual?  Can you identify the roughness to one mag or the other?  Are your plugs massive or fine-wire?  Do they exhibit lead fouling (ref MikeOH)?  If they do, refining your leaning procedures should help, as described above.  If not, how old are your plugs?  Massive or fine-wire electrode?  Correct heat rating?  What do they ohm out at?  

You can solve this mystery.  Congrats on your C purchase, you’ll won’t be disappointed.     

[jaylw314]

2 hours ago, MikeOH said:

Rags beat me to it.  You need to lean VERY aggressively on the ground, just as he described.  It needs to stumble if you advance the throttle much beyond 1200 rpm.

As a reference for cruise, at 8000 I'm WOT, 2500 rpm, and leaned to 8.5-9.0 gph.

The description of how the problem goes away after maintenance, and then returns is what suggests plug fouling, but I'm not smart enough to claim something else couldn't be going on, however.

A trick I use for leaning is when you start to taxi, run the RPM's up to about 1200 RPM to get the plane rolling, then lean until it starts to drop.  Now you can pull the throttle for taxi, and you know you have just enough mixture to taxi up to 1200 RPM but not much higher.  If you're below 1000 RPM and you lean to RPM drop, the mixture is much more sensitive and it's easy to make the engine stumble, then you're usually jamming the mixture back in to keep the engine from dying and have to start all over.

Edited June 30, 2022 by jaylw314

[Shadrach]

A 500hr inspection on a Bendix is typically treated as an IRAN (inspect and repair as necessary).  “Repair kits” are available but it isn’t necessary to replace serviceable parts. Incidentally, It’s not unusual for Bendix S20/200 series mags to go through 500hr inspection without overhaul. I’ve had them go through inspection without needing any moving parts.  They are solid mags (the best for your application in my opinion).
 

Ground leaning may help. If it does, you may need to have the idle mixture adjusted. I ground lean out of habit but my plugs would not foul if the engine was run full rich on the ground. Ground leaning is good practice (especially with carbureted applications) but it should not be necessary under normal circumstances, if the fuel system is set up properly. 

[A64Pilot]

You should not have to lean on the ground.

But if you choose to, lean the snot out of it, lean it so much that it won’t run above 1800 RPM or so. You cannot hurt an engine leaning it on the ground, but people have been killed taking off leaned out because they forgot, and of course it’s not in the before takeoff checklist, so that’s why I say lean it so much that you can’t attempt a takeoff.

[SessionDrummer]

This is an interesting discussion. Thank you.

In all candor, I THOUGHT I was already leaning pretty aggressively for taxiing. And, now if I'm understanding correctly, a couple of you are suggesting that it shouldn't matter (at least in the context of resolving whatever this issue is).

Here's another clue: After flying (with all the requisite leaning in cruise), I'll land and then do a run-up to see if anything has changed. Sometimes yes, sometimes no. That has been part of the frustration. It's inconsistent.

[MikeOH]

24 minutes ago, SessionDrummer said:

This is an interesting discussion. Thank you.

In all candor, I THOUGHT I was already leaning pretty aggressively for taxiing. And, now if I'm understanding correctly, a couple of you are suggesting that it shouldn't matter (at least in the context of resolving whatever this issue is).

Here's another clue: After flying (with all the requisite leaning in cruise), I'll land and then do a run-up to see if anything has changed. Sometimes yes, sometimes no. That has been part of the frustration. It's inconsistent.

If you get a bad run-up, have you tried to clear by leaning for a couple of minutes?

You could also try pulling plugs right after a bad run-up to look for fouling. 

[Ragsf15e]

1 hour ago, SessionDrummer said:

This is an interesting discussion. Thank you.

In all candor, I THOUGHT I was already leaning pretty aggressively for taxiing. And, now if I'm understanding correctly, a couple of you are suggesting that it shouldn't matter (at least in the context of resolving whatever this issue is).

Here's another clue: After flying (with all the requisite leaning in cruise), I'll land and then do a run-up to see if anything has changed. Sometimes yes, sometimes no. That has been part of the frustration. It's inconsistent.

There’s a setting in the engine for idle mixture adjustment.  If it’s not perfect, you can be too rich at idle.  If leaning more solves it and you want to dig deeper, that might be a place to start.

In my experience (slightly higher altitude airport), I need to lean on the ground no matter what.

[Ragsf15e]

1 hour ago, A64Pilot said:

You should not have to lean on the ground.

But if you choose to, lean the snot out of it, lean it so much that it won’t run above 1800 RPM or so. You cannot hurt an engine leaning it on the ground, but people have been killed taking off leaned out because they forgot, and of course it’s not in the before takeoff checklist, so that’s why I say lean it so much that you can’t attempt a takeoff.

This is a good point about takeoff.  I typically don’t use full rich for runup, and I relean after runup, but you want to make damn sure you’re full rich (or leaned appropriately at high altitude airports) when you takeoff.

Leaning during climb is ok if you know how.  Easier with an engine monitor.

[Kevin M]

I just went through the Mag debacle with my “new to me” 63 C.   It would exhibit the same type of events you described. My mags came back with a clean bill of health after so called rebuild.  Once the engine was hot (within upper limits)  it would misfire and shake like a leaf on a tree.  Hammdo lent me another set and it runs great.   On two occasions when it was really hot outside (>34c) ambient. It stumbled and seemed I was back to square one. I went ahead and had Tempest Massives installed and haven’t looked back.  Even the CHT gauge reads a little cooler than before during ground ops, climb, and cruise.  My previous set of plugs looked great. Where gapped properly and appeared perfectly servicable. I only replaced them so I would have a ground zero starting point for trouble shooting purposes.  Ground zero is all it took to consistently run as it should. It doesn’t seem as sensitive to ground leaning procedures as before the plug change.  I’m not a mechanic, nor am I even close to being a Mooney expert.  I just wanted to share my experience with you. 
Kevin

[jaylw314]

1 hour ago, SessionDrummer said:

This is an interesting discussion. Thank you.

In all candor, I THOUGHT I was already leaning pretty aggressively for taxiing. And, now if I'm understanding correctly, a couple of you are suggesting that it shouldn't matter (at least in the context of resolving whatever this issue is).

Here's another clue: After flying (with all the requisite leaning in cruise), I'll land and then do a run-up to see if anything has changed. Sometimes yes, sometimes no. That has been part of the frustration. It's inconsistent.

Without an engine monitor, you're kind of reduced to running up, and if you get roughness, shutting down the motor, opening the cowl and shooting each cylinder with an IR thermometer.  That might identify if one of the cylinder's is firing abnormally.

Did the mags actually get bench tested?

10 minutes ago, Ragsf15e said:

There’s a setting in the engine for idle mixture adjustment.  If it’s not perfect, you can be too rich at idle.  If leaning more solves it and you want to dig deeper, that might be a place to start.

In my experience (slightly higher altitude airport), I need to lean on the ground no matter what.

I do do my runup at full rich, but that's mostly an extra reminder to do so before takeoff

 

[Boilermonkey]

We had a similar issue and it turned out to be the carburetor.  It apparently was so old the OH shop said they sent it to a museum and gave us a reman'd one.  There's a lot that can get messed up that can lead to a rough running engine.  For ours, it had some similar behavior and it could sometimes stumble when going to full power.

[cliffy]

If leaning on taxi doesn't solve the issue put in a new set of Tempest massives at the hot end of the heat rating range of the plugs 

[MB65E]

Fine wires fowl easily. I like 37BY’s. Balancing experts say fine wires run smoother. (Yeah when it’s not fowled)

Every now and again I try a set of fine wires and I’m reminded about why I use the BY’s. 
Lean aggressively, keep your mags in good shape. (Sounds like you are)

-Matt
 

[SessionDrummer]

Update: The aggressive leaning procedure seems to be working. I thought I was leaning aggressively before, but evidently it wasn't aggressive enough. I'll be curious to see if this proves to be the entire issue, or if there's something else going on.

[Ragsf15e]

12 minutes ago, SessionDrummer said:

Update: The aggressive leaning procedure seems to be working. I thought I was leaning aggressively before, but evidently it wasn't aggressive enough. I'll be curious to see if this proves to be the entire issue, or if there's something else going on.

Done right after start and immediately after exiting the runway on landing, you should go a full year without a fouled plug.  They’ll be cleaned, gapped, checked at annual.

Hopefully it was actually the issue because it’s the easiest to solve.