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Old 06-24-2013, 10:04 PM
steve91t steve91t is offline
Join Date: Nov 2010
Location: Huntersville, NC
Posts: 138
Default Interesting P-mag issue...

This one is a thinker...I'll try to keep it short while giving as much information as possible.

So a few months ago the weather was perfect for flying my dad's RV 7 to and from work. I was flying it every few days. It's an RV7 with an IO360 with P-mags and about 400 hours TT (I think). Mags had never been overhauled until recently. More on that later.

So, to make this post a little less boring, I'll include some pictures. Here is the plane freshly washed in Roanoke, VA minutes before heading home.

Flying home...beautiful flight.

It's running pretty **** good if you ask me. The true airspeed is a little lower than you might think, but that's because of the composite 3 bladed prop.

Now for the fun part. I park the plane at Lake Norman airport just north of Charlotte, NC for the night. The next morning I hop in and fire it up. I do my mag check and when I turn the left P-lead off, it bangs and pops in the worst way I've ever heard. Sounded terrible. EGT's went through the roof when I turned it off for any length of time. When I turned the right off, the RPM smoothed out and increased.

Fortunately there were some very nice mechanics on the field. After a full day of trouble shooting, they found that the right P lead was out of timing. They retimed it and all was well. I picked the plane up a few days later, flew it a short flight back to our home base and put it in the hanger. We sent the P mags out to be checked and they really didn't find anything wrong. Their suggestion was to have the left one rebuilt ($85) and the right one rebuilt along with a new circuit board just to rule out this happening again ($450).

We got them back and decided to install them today. We've never done this before but it seemed simple enough. We followed the instructions. Installed the mags, connected the proper plug wires, connected the plug. We set TDC by pulling a plug on #1, gently inserting a screwdriver to feel the piston movement, and rotating the prop till it stopped. We also found the TDC mark on the flywheel lined up with the seem in the case. We timed the mags by blowing into the tube. Got the green lights like we should.

We cranked up the engine and all seemed well, until we turned one mag off. 90 RPM drop on the left, 170 on the left with about a 150 degree increase in EGT. It also sounded bad with one mag off. We retimed the mags 3 times to make sure we had it right. No real change. The odd thing is that with both mags on, it sounds great. Smooth, no popping, nothing. It sounded like a perfectly happy engine.

So, to summarize....

Engine ran great
Overnight, right P-mag lost timing, but once timed, ran great again.
Mags rebuilt, nothing found wrong. Company has no idea what happened.
Now both mags have an unacceptable drop during mag check after rebuild.

We are confident that we set TDC correctly. But the tech guy even said if we were a couple of degrees off that we wouldn't even be able to notice it, implying that even if we missed it just barely, it would still be fine. But we didn't, we had the marks lined up.

We are at a loss. Why would they both have such a big drop? Before the rebuild, they would maybe lose 10-20 RPM. Almost no drop at all.

The tech guy insists that we have something else going on, like a fuel problem. I call BS on this because NOTHING changed. All we did was remove the mags and then reinstall the rebuilt ones. We didn't even remove plug wires. I am very confident that we have a timing issue, but I don't know why.

I'm sorry for such a long post. It's tough to keep it short and provide as much information as possible. If you guys have any insight as to what might be wrong, we are all ears. The good thing about this experience is that we are learning a lot about the airplane and how the systems work. The other good thing about getting stranded that one day in Lake Norman Airport is I got to see this...

Anyone redneckinize this old barn? Remember the barn scene in "Days of Thunder"?
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Old 06-24-2013, 10:17 PM
tkatc's Avatar
tkatc tkatc is offline
Join Date: Nov 2007
Location: NJ
Posts: 1,747

What kind of plugs are you running? And it runs rough on the upper plugs or lower plugs?
My ATC opinion is NOT an official FAA recognized opinion, so any advice you get from me is ONLY my opinion.

Track my RV7A!!

Bought my flying -7A
Building an -8! (Fuse)
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Old 06-24-2013, 10:34 PM
steve91t steve91t is offline
Join Date: Nov 2010
Location: Huntersville, NC
Posts: 138

NGK plugs, changed during the troubleshooting process (.5 hr flight on them). Right mag is lowers, left is uppers. Runs rough on one or the other, but when both mags on, seems fine. Of course we never got it higher than about 1400 RPM.
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Old 06-24-2013, 10:42 PM
Flyguytki's Avatar
Flyguytki Flyguytki is offline
Join Date: Jun 2007
Location: Fairbanks AK
Posts: 758

What about plug wires, if the mags work (should be just overhauled), plugs are brand new only logical problem would be maybe one of the wires is bad.
RV-7 N87DX Built, Flown, and Sold!
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Old 06-24-2013, 10:54 PM
steve91t steve91t is offline
Join Date: Nov 2010
Location: Huntersville, NC
Posts: 138

One bad wire would cause a big drop with both mags? No way.

How would a plug wire go bad on its own? You see the screen shot of the engine gauges the day before the mag lost its timing. There was nothing wrong.

It's got to be either something with the mags or something with setting TDC.
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Old 06-24-2013, 11:15 PM
tim2542 tim2542 is offline
Join Date: Aug 2009
Location: Redding,Ca
Posts: 633
Default Random ideas...

Well for some ideas I'd suggest you put an ohm meter on every plug wire. I believe they give you the resistance per foot in the manual, but they should be all pretty close to each other. You said you put in new plugs, also make sure they are solid tops, the screw on ones can missfire if they come loose, had that problem myself.
Second, test the manifold pressure plumbing. If one is getting vacuum and the other isn't that would account for some difference. You can put an automotive timing light on #1 cylinder and see where it is firing, then check the other system on the same cylinder....they should be the same. Check it again with the vacuum disconnected and plugged back to the engine.
Bottom line is this: with 8 good wires and plugs, both systems firing at the same time, it has to be elsewhere. We should be able to assume the engine is mechanically sound since it runs fine on both.
Another thought is you should be able to see a bad plug or wire from a EGT rise that's different from the others when you kill on side or the other.
Final suggestion is swap the PMAGs from side to side and see if your troubles move also.

Last edited by tim2542 : 06-24-2013 at 11:21 PM.
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Old 06-24-2013, 11:22 PM
ao.frog ao.frog is offline
Join Date: Sep 2006
Location: Manstad, Norway
Posts: 866
Default Ohm?

Have you ohm'ed the plug wires?
The allowed ohm pr foot is described in the P-mag manual.

Maybe one wire has gotten a loose connection in a booth during the P-mag swap? Or maybe a wire is broken?

It's easy to yank the wires alittle because of the tight space when off- or on mounting the P-mags.

Brad at E-mag is VERY helpful and they truly are standing behind their product, so I'm sure he'll help you in any way he can.
Regards Alf Olav Frog / Norway
First RV-7 completed, (bought partly finished from a US-builder) 305 hrs per July 2014, SOLD
Second -7 had first flight Feb 25th 2014. 220 hrs pr July 2019. Life is good!
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Old 06-24-2013, 11:47 PM
steve91t steve91t is offline
Join Date: Nov 2010
Location: Huntersville, NC
Posts: 138

Thanks for the suggestions. But I don't think it's the wires. Think about it, how would all 8 wires all go bad at the same time. Like I said, both mags run like **** on their own. That's not a single plug wire issue.

I forgot to mention that the EGT rise while running on one mag was across the board.

As for the manifold plumbing, that's an easy one. They are T'd into the same source. All we did is unplug the line from the mags. We simply plugged them back in when installing.

I'm not trying to be difficult in any way, but think about it. Nothing changed other than the mags. We aren't dropping a cylinder on a mag check, that's easy to identify. That would be what happens with a bad wire.

It's acting like the timing is retarded when doing a mag check. BOTH mags respond the same way.

This is not a situation where we have a big drop with one only.

Thanks again guys.
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Old 06-25-2013, 12:00 AM
ao.frog ao.frog is offline
Join Date: Sep 2006
Location: Manstad, Norway
Posts: 866

A logical nest step would be to replace one P-mag at a time, just to see if the problem is with one or both P-mags.

Maybe one at your airport has a P-mag you can borrow?

Or maybe Brad can send you one to try out for a couple of days?
He's a great guy so it can't hurt to ask...?
Regards Alf Olav Frog / Norway
First RV-7 completed, (bought partly finished from a US-builder) 305 hrs per July 2014, SOLD
Second -7 had first flight Feb 25th 2014. 220 hrs pr July 2019. Life is good!
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Old 06-25-2013, 12:17 AM is offline
Join Date: Feb 2005
Location: Zurich, Switzerland
Posts: 475

did you make sure that none of the spark plug wires are swapped on one mag?
it may run on 2 cylinders only on one side, with the other 2 cylinders firing at the wrong stroke.
Bernie Daenzer, Alex Lichtensteiger
S/N 72072, Flying!
HB-YMT (Switzerland)
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