Intermittent left mag problem (AA5 - 0320)

francisco collazos

francisco collazos

Line Up and Wait
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Jun 11, 2021
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ciscovet
Good morning everyone and thanks in advance for your help. First let me preface this by saying that I am not a mechanic but I will try to provide a concise overview on what is going one here because I am at my ends witt with this whole situation. I do not approve fixs to the plane since i am not a mechanic.

I am part owner of a 1972 Grumman traveler with a 160stc and powerflow exhaust (rebuilt engine with about 600hr) and have had nothing but issues after the last annual which took place last august. At the time the left mag was rebuilt as part of the annual. The Rt is an electronic mag with low hours. The symptom is pretty much rough running when the mag switch is switched to left.
The left mag was thought to be the problem and sent out for rebuild twice.
A new ignition harness was bought which did not solve the problem
New Plugs (tempest) were bought and installed.

The odd thing is that a lot of times the motor has worked for a while after these replacements.
So here recently, they noticed a fouled plug on the number 4 cylinder and saw glazing of the cylinder so they decided to repair the cylinder and was just recently installed last week. I guess there was some increased oil consumption but I don't really know the exact amount. The plane was taken up last wednesday and it flew fine. We went to fly the plane this saturday and back to the same problem. It was even leaned aggressively and it did not solve the problem. Now the mechanic is saying the plugs might be bad which I don't believe.
I'm over this, like I said I don't know because im not a mechanic but i'm medical professional and I like to test things to get some sort of diagnosis not just replacing parts. I don't feel like anything has been done to even isolate what cylinder it is. I told the other members that we should get someone new involved here because we are thousands in on this problem and frankly we have replaced everything.
I will say we do have a nice CGR units so if there are any tests that I can do, let me know. I do remember nothing out of the ornery with the CHT/ EGT at the time.

any ideas??
 
What happens to your individual cylinder EGTs when you isolate each mag?

Can you do a data dump and compare a log before it was problematic (before your annual) and more recently?

Is the ignition switch wired properly to shut off the correct mag?
 
Post Flight Run -up & Mag Checks can alleviate plug fouling problems

and also avoid cancelling flights. I agree with diagnosing before launching

into any type of repairs but it’s a little late to say that. It seems basic

troubleshooting was not accomplished. “Cold Cylinder Check” and run-up

with left mag P-lead disconnected would get you to isolate whether the

culprit is 1 cylinder or in the mag.
 
Magman said:
It seems basic

troubleshooting was not accomplished. “Cold Cylinder Check” and run-up

with left mag P-lead disconnected would get you to isolate whether the

culprit is 1 cylinder or in the mag.
Click to expand...
Yes it's rediculous tbh... I have a feeling its going to be one particular cylinder but IDK.
Bell206 said:
Have someone troubleshoot the entire mag wire circuits to include the switch before replacing any more parts.
Click to expand...
Yes that's going to be the plan when this second set of spark plugs does't work
455 Bravo Uniform said:
What happens to your individual cylinder EGTs when you isolate each mag?

Can you do a data dump and compare a log before it was problematic (before your annual) and more recently?

Is the ignition switch wired properly to shut off the correct mag?
Click to expand...
1. IDK - i am just looking for ideas so maybe I can do some diagnosing myself.
2. good question - IDK but i will look into it
3. ONce again good question I will look into it.

I appreciate the replies. One question is..in what scenerio can it be the engine and not the electrical since it happens consistently on the left mag switch -off?
 
It could be a bad mag, a bad plug, bad ignition wire, bad P-lead, bad switch. First thing I'd try is to swap the fouled plug (after cleaning it, of course) with the other plug in the same cylinder and see if the problem goes to the other side, or if the same plug fouls in the new location.

A bad mag or plug wire could result in a weak spark and thus fouling, with the roughness not happening until it has time to foul.
 
Numbers on “ rough running”?

A “ Mag Check “ is not simply Ignition. It is comparing the quality of combustion in the different cylinders. An Intake Leak on one cylinder
may cause a Mag Drop , sometimes. Generally rough running is indicative of
dropping one cylinder.

Suggest you try Lycoming’s Shut Down Procedure. Not Grumman.
After flight run up to about 2000 rpm for 30 seconds. Good time to
check Mags. Reduce power to 1000 and go to cutoff. Do this when clear
of other aircraft.

“ happens Left Mag switch-off”? My interpretation of that comment is you’re running on the Right Mag. ??
 
Magman said:
A “ Mag Check “ is not simply Ignition. It is comparing the quality of combustion in the different cylinders. An Intake Leak on one cylinder
may cause a Mag Drop , sometimes. Generally rough running is indicative of
dropping one cylinder.
Click to expand...
excellent good to know that makes sense now.
Magman said:
“ happens Left Mag switch-off”? My interpretation of that comment is you’re running on the Right Mag. ??
Click to expand...
correct. the rt is a surefly.
 
Last line of #5 says “ Left Mag switch-off” would indicate you are running

on Right (Surefly) and associated plugs , harness etc?


Communication is essential. Folks have been known to change incorrect

mags , engines etc. I was pax in a C-47 ( DC-3) and the IP pulled LEFT

Mixture. He then said “ make sure you feather the RIGHT

( meaning CORRECT) one. Student feathered the RIGHT one!

It was exciting for a while!
 
Was that left mag rebuilt according to the magneto manufacturer's service manual? There is too much work being done ad-lib and stuff isn't done right. If the overhauler put too much oil or grease on the cam lubrication, that stuff gets thrown into the points and fouls them, making for poor/intermittent contact and rough running. Or he didn't set the E-gap properly and the spark is weak. Or he didn't get the mag's internal distributor gearing properly indexed. Or the condenser wasn't replaced and is weak. So many ways to screw up.
 
Dan Thomas said:
Was that left mag rebuilt according to the magneto manufacturer's service manual? There is too much work being done ad-lib and stuff isn't done right. If the overhauler put too much oil or grease on the cam lubrication, that stuff gets thrown into the points and fouls them, making for poor/intermittent contact and rough running. Or he didn't set the E-gap properly and the spark is weak. Or he didn't get the mag's internal distributor gearing properly indexed. Or the condenser wasn't replaced and is weak. So many ways to screw up.
Click to expand...
yeah twice!!! IS it possible that the leads are reversed and when the left mag is set to off the right mag is what is shut down and we are looking at the wrong mag? The other mag is a surefly, what;s the chances of that?
 
francisco collazos said:
yeah twice!!! IS it possible that the leads are reversed and when the left mag is set to off the right mag is what is shut down and we are looking at the wrong mag? The other mag is a surefly, what;s the chances of that?
Click to expand...
This is confusing. When the left mag is shut off, you're running on the right mag. You said the right mag is the surefly. So which one is having the problem?
 
If I’m reading this correctly, it seems like everyone is focusing on the left magneto and messing around with it when they should be trying to figure out why the Surefly is making it run bad.
 
Lindberg said:
This is confusing. When the left mag is shut off, you're running on the right mag. You said the right mag is the surefly. So which one is having the problem?
Click to expand...
I'm sorry, its early and I haven't had my coffee haha. So yes the issue is when the Mag switch is set to (L) so the left mag is active and that's when we have the problem. The rt is a Surefly. What i was trying to say is if it was possible to change the wires so the mags are reversed.
 
francisco collazos said:
What am I looking for here?
Click to expand...
A post-flight run-up is actually what's recommended by Lycoming to keep the plugs clean, particularly if you have a long taxi back to your parking spot. Doing a mag check during that run-up can tell you if the engine behaves differently when everything is warmed up.
 
Beat me to the answer. The higher power promotes de-fouling plugs

of lead and contaminants. It can be very effective. It also alerts you that

you may may need maintenance before next flight.


Most flights are configured that there is a gradual reduction in power during

the descent. This power reduction reduces the temperature in the

combustion chambers and allows lead to accumulate and foul plugs.

If I understand your comments the issue can be corrected by cleaning

the plugs? It could be possible incorrect plugs are installed. I’m a fan of

(U)REM 37BY plugs if they are approved for any engine. They were designed

to minimize the effects of lead fouling.

Dan asked for details on the Left mag work. There are procedure that need

to be followed. Even the type of lubricant used is important and not

everyone has it.
 
francisco collazos said:
I'm sorry, its early and I haven't had my coffee haha. So yes the issue is when the Mag switch is set to (L) so the left mag is active and that's when we have the problem. The rt is a Surefly. What i was trying to say is if it was possible to change the wires so the mags are reversed.
Click to expand...
Yes.
 
Slick and Bendix have entirely different ways of adjusting E-gap.

Tolerances on Contact Points is also different.

Simple tools and varying techniques are needed for each brand.

Cold Cylinder Check is long overdue as it sounds like the mag is NOT the

culprit.
 
1 Fouled plug = firing 3 cylinders= roughness

You have to do the check when the issue presents.

No need to be A&P as no disassembly.

Laser thermometer would help but not essential.
 
Dan Thomas said:
Sure, twice rebuilt without doing it according to the manual, and you will have problems twice. Twice does not guarantee that it was done right the second time.
Click to expand...
As evidence, I had a rebuilt alternator installed on my newly overhauled engine which kept tripping the breaker.
It had been done by a well-respected, widely-known aviation electrical shop.
(Lots of troubleshooting later) the culprit was the alternator. Sent it back for inspection & testing- they found no problems even after hours of test running. They changed a few parts.
Reinstalled, same result.
Sent back again; they again found no problem so they rebuilt it.
Reinstalled; immediate and long term resolution of problem.
They were perplexed to the end.
 
Report CGR info. And probably get a new mechanic - too much shotgunning at this point. FWIW have often started to buy new mags if the old ones are getting too tired.
 
So I want to thank everyone that helped me with this problem. We went back to basics and switched the mag with another one and it ran no problem. So this mag has been sent to be rebuilt twice. The first time last year during the annual and after the annual is when all the problems started. I was under the impression that the mag would be shipped to the factory but I guess I was wrong and there's a person here in Florida that does the rebuilds. I told my partner to keep the other mag on the plane for a while to make sure it's truely the problem.
 
Sometimes when you try to save money, you don't save money. I usually replace my mags with new when they fail, and send in the old one for somebody else to use. Even new ones have issues. I got hit with an AD to replace apparently defective distributor arms on my new mags. So far, so good, but we are approaching the 500 hour mark at this point...
 
Another reminder that the status of Service Bulletins could be verified

prior to installation. Preferably prior to purchase. I am referring

to NEW mags here!
 
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