Mag check goes "pop"

[Mcstealth]

While testing our new cylinders on an O-320, 150hp Lycoming, the first click on the mag check the engine went pop pop pop, but did not drop rpm under acceptable levels. In fact, the rpm drop was just 75 or so. Second click was no popping with normal rpm drop. Is that a timing issue?

[N201MKTurbo]

Something is wrong with your mag. It may be cross firing. It wouldn't hurt to check the timing and I would look at the point gap.

[EricJ]

I'd also check that the ignition plug wires all go where they're supposed to.

[Mcstealth]

12 hours ago, EricJ said:

I'd also check that the ignition plug wires all go where they're supposed to.

Please expound if you don't mind. 

[SKI]

I think Eric meant make sure the #1 wire from the mag does in fact go to the #1 cylinder ect for all cylinders. That was my initial thought as well.

[EricJ]

2 hours ago, Mcstealth said:

Please expound if you don't mind. 

Make certain that the #1 ignition wire from the mag goes to the proper plug on the #1 cylinder, #2 wire goes to number 2, etc.   Basically, that the wires are routed properly.    If they're not you can get an engine that runs but backfires.   It's just something to verify.    These days most harnesses are marked near the plug with the cylinder number and plug position, so it may just be a quick inspection check.   Otherwise you may have to trace them from the mag, which is tedious but doable.

[Mcstealth]

On 12/14/2023 at 9:54 AM, EricJ said:

Make certain that the #1 ignition wire from the mag goes to the proper plug on the #1 cylinder, #2 wire goes to number 2, etc.   Basically, that the wires are routed properly.    If they're not you can get an engine that runs but backfires.   It's just something to verify.    These days most harnesses are marked near the plug with the cylinder number and plug position, so it may just be a quick inspection check.   Otherwise you may have to trace them from the mag, which is tedious but doable.

The A/P who put it all back together is sticking to his guns and saying we need to adjust the carb saying it is too rich. Carb is new, less than 50hrs on it, FYI. 

My brain says if it is too rich, then it would make the popping regardless of the position of the mag check. 

[Fly Boomer]

2 minutes ago, Mcstealth said:

The A/P who put it all back together is sticking to his guns and saying we need to adjust the carb saying it is too rich. Carb is new, less than 50hrs on it, FYI. 

I thought you said it only does this on one of your mags.  How does the carburetor know which mag you are on?

[Mcstealth]

37 minutes ago, Fly Boomer said:

I thought you said it only does this on one of your mags.  How does the carburetor know which mag you are on?

Correct. Only one mag. That is why I am skeptical of the A/P's reasoning. 

Engine runs smoothly in normal operation, or atleast it seems so. I do not hear any rouge detonation. Only pops on the first click during run up. 

[skykrawler]

The "first click" from the "BOTH" switch position means you are operating on the right magneto.    You say new cylinders.  Are the plugs new and gapped correctly?  Generally, if a plug/cylinder misfires raw gas vapor is pushed into the exhaust.  The next stroke will ignite it and it will backfire.

If the wires are crossed up or the mag 'cross firing' the spark is mistimed.

Its easy to lean without adjusting the carburetor - there's a knob.

 

[A64Pilot]

On 12/18/2023 at 4:19 PM, Mcstealth said:

The A/P who put it all back together is sticking to his guns and saying we need to adjust the carb saying it is too rich. Carb is new, less than 50hrs on it, FYI. 

My brain says if it is too rich, then it would make the popping regardless of the position of the mag check. 

It would also stop if you leaned it.

‘It’s embarrassing but I have swapped plug wires on my C-85 for one mag between 1&3 and it did pretty much what your saying, it may have had more drop though

[Mcstealth]

20 hours ago, A64Pilot said:

It would also stop if you leaned it.

‘It’s embarrassing but I have swapped plug wires on my C-85 for one mag between 1&3 and it did pretty much what your saying, it may have had more drop though

Okay. Finally got the A/P cornered and I have pulled the cowl. The left mag has all four of the bottom plugs. The right mag has all the top plugs (obviously).

Edited January 12 by Mcstealth

[Mcstealth]

And I am still slightly unclear. The first click to the "R" position means we are grounding out the right mag and running solely on the left?

[Hank]

29 minutes ago, Mcstealth said:

Okay. Finally got the A/P cornered and I have pulled the cowl. The left mag has all four of the bottom plugs. The right mag has all the top plugs (obviously).

On my Mooney, one mag runs the Lower Left and Upper Right plugs, the other runs the Upper Left and Lower Right plugs.

Each mag should run upper and lower plugs on opposite sides. When the wiring harness was replaced, the ends were labeled this way.

[Hank]

27 minutes ago, Mcstealth said:

And I am still slightly unclear. The first click to the "R" position means we are grounding out the right mag and running solely on the left?

I thought "R" grounded the left and ran the Right magneto.

[Pinecone]

18 minutes ago, Hank said:

On my Mooney, one mag runs the Lower Left and Upper Right plugs, the other runs the Upper Left and Lower Right plugs.

Each mag should run upper and lower plugs on opposite sides. When the wiring harness was replaced, the ends were labeled this way.

This is normal with two mags.

With one electronic, they recommend that the electronic fire all the lower plugs, and that they use fine wire.

[Mcstealth]

I'm such a duffus. I got lazy, didn't do my research, and now will do double work becase of it. I discovered the A/P had one mag configured on all the bottom plugs and vice-versa. This resulted in the crossfiring several of you had mentioned. A little research (Thanks @LANCECASPER) shows me I swapped the wrong side of the engine. I switched 2&4 top to bottom where I should have done 1-3. Back to the hangar tomorrow but atleast it didn't backfire when we tested her. 

Edited January 15 by Mcstealth

[skykrawler]

I don't think having the mags configured all bottom/all top will cause a problem and make the engine misfire.  Its just a practice - but I don't really understand the rationale.  Something like if 'top' mag fails you don't have the 'bottom' mag trying to fire through all the oily/leaded up bottom plugs.

[A64Pilot]

7 hours ago, skykrawler said:

I don't think having the mags configured all bottom/all top will cause a problem and make the engine misfire.  Its just a practice - but I don't really understand the rationale.  Something like if 'top' mag fails you don't have the 'bottom' mag trying to fire through all the oily/leaded up bottom plugs.

It shouldn’t and we as in A&P’s aren’t really supposed to just do things like reconfigure Mag firing on our own. Now if there is some kind of approved data like an STC for an Emag or something then we have to follow it, otherwise it’s supposed to be wired IAW the engine manual.

One magneto fires negative and the other positive, meaning that one wears the center electrode and the other the side electrodes. Therefore there is a rotation order you follow for plugs to even the wear and make them last longer.

https://www.aopa.org/news-and-media/all-news/2019/september/05/aircraft-maintenance-become-a-spark-plug-guru

However I do not think the rotation applies to us with (D) as in single drive mags, I think but am not positive that both of them fire the same polarity. If so rotation won’t hurt of course just won’t help either

I don’t have a clue about electronic ignitions

[A64Pilot]

On 1/12/2024 at 1:46 PM, Hank said:

I thought "R" grounded the left and ran the Right magneto.

That’s my understanding as well 

[Mcstealth]

On 1/12/2024 at 12:46 PM, Hank said:

I thought "R" grounded the left and ran the Right magneto.

I am not sure one way or the other. Time for me to do some research. 

[skykrawler]

16 hours ago, A64Pilot said:

It shouldn’t and we as in A&P’s aren’t really supposed to just do things like reconfigure Mag firing on our own. Now if there is some kind of approved data like an STC for an Emag or something then we have to follow it, otherwise it’s supposed to be wired IAW the engine manual.

 

Didn't say it should be changed.  Just saying if it's done as he said, that it doesn't explain away the problem he was having.

I do understand the polarity difference and why plugs should be rotated.

One explanation for top/bottom for each magneto is so that the rpm drop for both mags will be similar.  I suppose when running on bottom plugs only the RPM drop would be more due to oil/lead fouling.

[A64Pilot]

22 minutes ago, skykrawler said:

Didn't say it should be changed.  Just saying if it's done as he said, that it doesn't explain away the problem he was having.

I do understand the polarity difference and why plugs should be rotated.

One explanation for top/bottom for each magneto is so that the rpm drop for both mags will be similar.  I suppose when running on bottom plugs only the RPM drop would be more due to oil/lead fouling.

I don’t know and there are things with mags that I can’t explain.

For instance I believe on my 1946 C-85 the left mag fires only the lower plugs, the right the upper, and they are timed 2 degrees different with the lower plugs firing first. It gives different mag drops between the two mags too.

The supposition is the mixture is leaner near the exh valve and that leaner mixtures flame front is slower and the 2 degrees makes up the difference.

Point I guess being that even 100 years ago they weren’t stupid and had these motors figured out, nothing in truth is new and we shouldn’t go changing things assuming there won’t be any difference.

I’m not inferring you did or anything, just saying as A&P’s we aren’t allowed to not follow the manual, and sometimes there are reasons for things being the way they are that we don’t know about and we could cause unintended consequences if we go mucking about with things.

Besides one has to question someone who says a miss on one mag is from mixture and says the carburetor needed to be leaned out?

Makes you wonder how he meant to lean the carburetor out? Only way I know is by changing the main jet and or venturi, the screw adjustment just changes low speed (idle) mixture.

 

 

[Mcstealth]

3 hours ago, A64Pilot said:

 

Makes you wonder how he meant to lean the carburetor out? Only way I know is by changing the main jet and or venturi, 

My thoughts also. The popping was at the top side of the run up at 1800rpm.