Home > VansAirForceForums

-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

Old 01-02-2021, 07:31 PM
Lt Dan Lt Dan is offline
Join Date: Dec 2014
Location: Utah
Posts: 94
Default Lean of Peak Problems with Dual P-Mags

This is the second of two issues I'm having with my P-Mags on my RV-8 with a Lycoming IO-360-M1B. I've been running dual P-Mags ever since I bought the airplane about 150 hours ago. The other issues is erratic RPM readings, which I posted about separately, so as not to cross streams. I believe this to be completely unrelated.

It has recently become harder to run lean of peak. As I approach peak EGT, the engine seems to develop a slight stumble. It's not what I would describe as roughness, but rather an almost rhythmic stumble. Today I attempted Mike Busch's LOP mag check in flight. I ran lean of peak and alternately switched off each mag. The engine ran very rough and exhibited lots of popping on each of the left and right mags when running individually.

I thought that the problem might have been old spark plugs getting weak, as I only recently learned to replace my automotive plugs after about 100 hours. The old ones had approximately 150 hours on them and I just replaced them with new NGKs: BR8ES Part Number 3961, but alas, the engine acts the same with the new plugs. I gapped them all to between 0.032 and 0.035. Any ideas on what might be causing this behavior?

Thanks in advance for the help!
‘08 RV-4
‘09 RV-8
Reply With Quote
Old 01-02-2021, 08:11 PM
Full Throttle Full Throttle is offline
Join Date: Aug 2018
Location: Norwalk, CA
Posts: 58

Try gapping the plugs at .022-.025 and see what happens. It may very well help.
Reply With Quote
Old 01-02-2021, 08:27 PM
Ironflight's Avatar
Ironflight Ironflight is offline
VAF Moderator / Line Boy
Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,597

The only times I have noticed a bit of stumble with my P-Mags I traced it down to a deteriorating plug wire - check the resistance (per the manual) on all the wires and see if that might be the issue.

Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
Reply With Quote
Old 01-02-2021, 08:36 PM
Gash's Avatar
Gash Gash is offline
Join Date: Aug 2009
Location: Goodyear, Arizona
Posts: 923

Hey Lt Dan, I've been a dual PMAG user for about 1500 hours on 3 different planes. Here are some ideas:

1. Time your mags. It's easy (blowing in the tube) and takes about 5 minutes. Follow the instructions in the user manual.

2. Check your fuel flow. Your PMAGs could be perfectly good, but you're simply not leaning to where you "think" you are because of invalid fuel flow indications. It wouldn't hurt to review your fuel flow indicator manufacturer's instructions on how to check and re-calibrate the K factor if it's wrong.

3. Are you using a Red Cube fuel flow transducer with your IO-360-M1B? They have been known to slowly fail (it's happened to me twice). They don't just die, but they become slowly inaccurate for awhile and then one day they'll just start sending a totally erratic signal.

4. I recommend following the PMAG user manual's spark plug gap recommendation. I also ran the NGK BR8ES plugs in my IO-360 engines, and I always gapped to .032. I never had a problem with this. For reference, I could always run those engines LOP at ~7.5 GPH, WOT and they were as smooth as silk.

5. A suggestion for your erratic RPM reading (if you're using a digital tachometer): See pages 4-6 of the PMAG Installation and User guide. Wire 6 in the control lead plug is your digital tach signal. If it's loose in the control lead plug, then you could get an erratic tach. Also check the 2 control plug anchor screws. If those are loose, the plug can work loose and give you tachometer (and other) problems.
Karl, Goodyear, Arizona (KGYR) ATP, CFII
Yak 50
=VAF= donor 2021
Reply With Quote
Old 01-02-2021, 09:22 PM
Simon Hitchen's Avatar
Simon Hitchen Simon Hitchen is offline
Join Date: Apr 2006
Posts: 298

I run two p-mags also and can lean to 100 LOP perfectly smoothly. You mentioned Mike Bush. If you pay for a subscription and have an EMS, download the leaning data and let his team diagnose the issue. They really know what they're doing. Your ignition should easily pass the leaning health test, especially with new plugs.

Maybe the previous owner timed the P-mags incorrectly. If a mechanic tried to time them like normal mags it would cause all sorts of issues. That's a good place to start.
Simon Hitchen
Port Perry, ON, Canada
7 Tip Up, Titan XIO-360, Dual P-Mags, Airflow Performance matched Injectors, Sensenich FP Prop, Dynon Skyview, GTR-200, GTX-327
Reply With Quote
Old 01-02-2021, 09:38 PM
Bevan Bevan is offline
Join Date: Sep 2007
Location: BC
Posts: 1,702

Meter the Pmag spark wires. They should all be in the same range (300-500 ohms as I recall). If you get some above 1000 ohms (or fluctuating when twisting and pulling on the ends) replace or take them apart and carefully replace the connectors. I’ve had some crimps at the plug end go bad causing high resistance. Engine was still running fine when I found the poor crimps.
RV7A Flying since 2015
O-360-A1F6 (parallel valve) 180HP
Dual P-mags
Precision F.I. with AP purge valve
Vinyl Wrapped Exterior
Grand Rapids EFIS
Located in western Canada
Reply With Quote
Old 01-02-2021, 09:55 PM
bruceh's Avatar
bruceh bruceh is offline
Join Date: Mar 2008
Location: Ramona, CA
Posts: 2,428

I had a similar issue. The tiny spring steel half circle clip on the end of the spark plug wire that holds the electrode was broken.

It is the piece on the right.

I found a new one at West Marine, and got it installed on the existing plug wire. I had to destroy the boot to access it. I got a replacement boot from E-mag.

I did the same inflight mag check and it pointed to Cylinder #1 and the top sparkplug. I knew something was wrong when the engine would just burble a bit here and there. Temps on the #1 cylinder were just a bit out of line from previous experiences. Only real indication was that the clip didn't feel tight on the end of the sparkplug, but with the rubber boot, it was difficult to really see what was going on until I cut off the boot.
Bruce Hill
RV-9A N5771H flown over 800 hours!
APRS Tracking for KJ6YRP and New Flying Blog
2020 VAF donator
EAA Tech Counselor, Build assistance - canopy/tanks/fiberglass/electrical/repairs
Reply With Quote
Old 01-03-2021, 12:24 AM
rv8ch's Avatar
rv8ch rv8ch is offline
Join Date: Feb 2005
Location: LSGY
Posts: 3,744
Default timing and plug wires

All good advice you have gotten here. I also had a bad plug wire. It showed up with sound in the headset and similar symptoms that you have.
Mickey Coggins
"Hello, world!"
Reply With Quote
Old 01-03-2021, 07:04 AM
pilotkms's Avatar
pilotkms pilotkms is offline
Join Date: Mar 2007
Posts: 470

Non-builder with apparent Pmag issues - have u:
1- obtained the build docs from VANs ?
2- wiring schematic from the builder ?
3- downloaded the Pmag install & maintenance doc from the Emagair website ?
U are going to need all of these. U need to learn how they work & how yours were wired. U may have to get a multimeter and trace wires. Are they both connected to manifold pressure with good connections ? U definitely need to re-time them (EZ!) so u know how to do it and how much advance they will be set for.
I suggest u do this first.
Recently did the GAMI lean test and changed one Nozzle restrictor for less than 0.2 gph differential. Works great.
RV 7A RV #9700 May 2017
N325KS the Flying “K”
Built in SoCal KCCB, now in GA @ KPXE
600+ Hours & 7X cross the USA
OSH flyin 2018 & 2019 & Petit Jean 2019
Reply With Quote
Old 01-03-2021, 04:40 PM
Lt Dan Lt Dan is offline
Join Date: Dec 2014
Location: Utah
Posts: 94

All great advice here. I appreciate it.

I just got done setting the timing. It was good, (maybe 3° past TDC) but now it’s perfect. Of note, I used the “crimp and pinch the tube” method (as mentioned on the E-Mag Air website) instead of blowing and it was MUCH easier!

I also just got done putting an ohm meter to every ignition wire and the results I got were rather shocking. So much so that I don’t know if this is actually possible. Two wires showed good resistance. ALL of the others showed open loops or nearly so. Could the engine have even run if this data is valid? Maybe my ohm meter couldn’t complete the circuit, but the thousands of volts the P-Mags are sending could get the job done (barely)? Is that possible?

The plugs that were in the engine previously were all NGK BR8ES’s with removable terminals and every one of them was coming loose. Is it possible that (as I have heard) these plugs caused arcing issues which fried my ignition harness?

By the way, upon inspection, I found in each of my cylinder 2 and 3 wires, one half of those of those clips as in Bruce’s post above. They were broken, very rusty and corroded, and falling out. Seems like this harness is toast.

I’m still not sure my resistance data could even be valid. I did make sure I had solid connections, correct voltmeter settings, etc.

Cyl 1
Length: 36 in
Nominal: 540 Ω
Actual: 517 Ω

Length: 40 in
Nominal: 600
Actual: 1 (Open Loop)

Cyl 2
Length: 40 in
Nominal: 600
Actual: 1 (Open Loop)

Length: 31 in
Nominal: 465
Actual: 4050

Cyl 3
Length: 29 in
Nominal: 435
Actual: 1 (Open Loop)

Length: 34 in
Nominal: 510
Actual: 1 (Open Loop)

Cyl 4
Length: 30 in
Nominal: 450
Actual: 1,046,000

Length: 26 in
Nominal: 390
Actual: 378
‘08 RV-4
‘09 RV-8

Last edited by Lt Dan : 01-03-2021 at 06:28 PM. Reason: Added broken clip data
Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -6. The time now is 10:00 PM.

The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.