Rough running mag.

henryinparadise

henryinparadise

Pre-Flight
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Oct 27, 2023
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Henry
Recently on a long cross country, after gassing up and running up,
the right mag dropped 300 rpm vs the left mag.
RV7 with a Lycoming O 360. (carb)
There was a very nice fbo who immediately helped me with the problem.
They: timed the mag. (it was fine). replaced 4 fairly new plugs, tore apart the mag
and found nothing wrong. the mag drop was unchanged.
so i limped on home. actually it flew fine.

im thinking a couple things here.
a bad ignition switch.
bad high voltage ignition wires.

any ideas?

thank you.
 
I had an intermittent rough-running mag a while back and it turned out to be the condenser wire was installed incorrectly and rubbing on the shaft screw. It took a couple hundred hours until it started shorting out and causing misfires. You said they "tore apart the mag," not sure what that entailed, but it would be something to check. Also, did they check the internal timing of the mag? How many hours until your recommended 500 hr mag inspection/IRAN?
 

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Way too often things are taken apart before the diagnosis is done.

YMMV but this is my approach.

1. Run the engine and be absolutely sure of which mag and amount of drop.

Since “ drop “ is about “Quality of Combustion” you may find that

leaning or Carb Heat may affect it.

2. Smooth drop is most often timing related. Rough drop is generally 1 or 2

cylinders. Cause is related to the secondary circuit. ie. Plug , lead or

distributor. Roughness occurs because that cylinder is not firing

properly.


3., Rough Drop can be diagnosed via instruments or a “ Cold Cylinder

Check”. Starting with a cold engine you switch to the bad mag

IMMEDIATELY after starting. Run at about 1000 rpm for 30 seconds.

Shut down and check for cold exhaust stacks. A laser temp gun works

great but a wet rag, water spray and a wet finger will tell the tale.

If you are unable to isolate a cylinder the you may want to pull the mag.

ALMOST ! You may want to determine if p- leads are correct first.

1 way is with a timing light. The other you can remove a lead and

see if you get spark corresponding to switch position. Some engines work

just fine with p- leads reversed.


I AM a believer in post flight run ups to check for drop and also burn off

deposits while the engine is hot. Lycoming has a SL on this topic.


Ignition Leads are best checked with a “Lead Tester” that will verify

the insulation around the conductor is intact and not “leaking”.

Continuity is seldom the issue here.
 
Coils will open up with heat and produce a rpm drop on a hot engine. When the engine is cool, drops are normal. Why you should do a mag check after every flight. Only shows up with a hot engine.
 
Mags fail while operating in flight but often are not discovered until

the next preflight run up. Why wouldn’t you want to have some assurance

it’s ready for next flight when you put it away?
 
Magman said:
Way too often things are taken apart before the diagnosis is done.

...I AM a believer in post flight run ups to check for drop and also burn off deposits while the engine is hot. Lycoming has a SL on this topic. ...
Click to expand...
I do it as part of my maneuver positioning for putting her back into the hangar. I need the high throttle to whip her ass around in a tight space. However, it's not the recommended minute. It's more like 20 seconds but higher than 1800 RPM. Maybe I'll dawdle a bit longer in the future. I haven't had a fouled plug in years. I service the plugs with every 50-hour oil change and lean almost to death on the ground preflight and somewhat less postflight.

Page 1, Para 5.

 
I'd consider running an "In-Flight LOP Mag Check" (assuming your O360 can run LOP passably well)... and upload data for analysis.
 
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Most flight are configured that there is a gradual reduction in power

accompanied by richening the mixture as air becomes denser. Low power

settings do do produce sufficient heat in the combustion chamber to allow

scavenging agents to reduce build up of deposits. Then you taxi in a low

power/ temps. Deposits stick to things like electrodes, valves, etc.

and cause problems. Many of the engines we fly were designed for

far lower lead content. There was a world before 100L that included

73, 91, 100/130. I pumped and burned a lot of 115/145.
 
henryinparadise said:
They: timed the mag. (it was fine). replaced 4 fairly new plugs, tore apart the mag
and found nothing wrong. the mag drop was unchanged.
Click to expand...
I'd want to know the details of that teardown. If they didn't reset the internal timing, well, there's your problem. If they don't know that they have no business going into a mag.
henryinparadise said:
so i limped on home. actually it flew fine.
Click to expand...
And what if the "good" mag had failed? You'd be going down to a forced landing. Your engine has two mags because (A) being electrical devices, mags are prone to failure, especially if they're not maintained according to their manufacturer's instructions, and (B) You get better power with two ignition sources. You will note that your engine has one carburetor, but two mags. That's because carb problems are really rare, while mags need proper care and attention. Taking off with one mag sick is asking for an accident.
 
I finally had time to look at the plane.
As per Magman, I started the engine and left it on R for 45 sec.
The #2 cylinder was cold. According to the wiring diagram, the right mag lead should go to #2 bottom. It does not it goes to #2 top.
I checked #2 top plug. It was suspect so I replaced it. But it still
runs cold! #2 bottom had been replaced earlier.

Ran out of time.

So the switch is wired wrong or the harness is wired wrong.
or the mag some how is faulty. It had an IRAN 130 hrs ago.
or the leads are faulty. ???

Also Dan is correct. I should not have flown it.
Especially over some very rugged mountains.
Like the Cascades by Mt Shasta.
 
So now you know the cylinder.

You can swap top n bottom plugs and see if drop changes ( then it’s the

plug) or stays the same ( then it’s a lead or bad distributor block in mag)

There are a few ways the harness can be made and installed.

As long as it goes from RIGHT mag to #2 cylinder you are ok.ERIFY

Most; not ALL, LYCOMINGS only have an Impulse Coupling on LEFT mag.

To verify switch wiring:

1. Remove ALL plug leads. ( for safety)

2.. identify RIGHT mag. ( CP side)

3. Take one lead from LEFT and hold spring end near a good ground .

Exhaust stack and 3/8 inch away.

4. Mag switch - LEFT

5. Have the engine cranked either manually or with Starter while you

observe if there is spark at the lead. Spark = switch wired correct

No spark unless switch put in RIGHT means wired incorrect.

1100 hrs. =. ??

Add at least 1 hour to replace harness.
 
Mag Man
if the switch is wired backwards, that doesnt necessarily disrupt or
bother the engine operation? does it?
i ask because the switch would be very hard to get at.

thanks H
 
There's an ohm spec for the plugs and wiring harness I run in my E-ab. Not sure what wires and plugs @henryinparadise is using or their respective ohm specs. My iridium plugs are around 5000 ohms and MSD wires are 50 ohms per foot. Might that test be useful here?
 
P-leads could be swapped at either end .

Very common on Cessna 150’s.

With a Lycoming you likely have an impulse coupling on the

LEFT mag only to retard spark when starting.

Switches are usually set up to shut off RIGHT mag to prevent

kickback that could damage Starter.

With time and patience you can troubleshoot this.

You may need a Tech if the mag needs to be addressed.

Some of these simple tests are often bypassed. This may have happened

in your case if things are miss wired.

I’ve met the “ Good” mag in pieces before I could address the “ Bad” mag!

Not quite as bad as some guys that changed the wrong engine on a

C-46. Left is facing the airplane?
 
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as Alice in Wonderland said. "curiouser and curiouser"

my ignition wiring is ass backwards. the reason it ran so well for
me for 2 yrs is that the switch was set up backwards too.

he must have thought left n right were facing the plane.

that is why the mechanic and i never showed any interest in the left mag or wires. the left wire to #2 cylinder was faulty. easy fix. i ordered new
harnesses which will be installed correctly and the p leads will be swapped then. hope that solves it.

learned something new.
that was an assumption most everyone would have had.
yet it was incorrect.
havent got the bill yet.
im sure it will be $600 on up which is what they earned and
will be paid.

thanks for all of the inputs esp. mag man
 
New harness is stupid expensive for what it is. But don't have many options to get it from.
 
I’ve have a few Slick and Champion harnesses I want to part with.

Need p/n on harness as I don’t know application.
 
i could not find a pn.

its a lycoming O-360-A1A
3/4/in nut
slick mags
sn 0616-SP

aircraft spruce was unable to locate the correct one.
 
I answered previous via dm.

Champion harness for Bendix mag

I have CH 12364 E for Bendix mag.

Suggest dm if interested.
 
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