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11-15-2014, 04:47 PM
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Join Date: Oct 2009
Location: Newark, IL
Posts: 287
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P-Mags - smooth on either, rough on both: SOLVED!
A week ago I posted a request for advice about a recent engine roughness issue. The IO-360 (C/S prop, dual P-Mag, AFP injection, 32 hours since new) had developed a distinct roughness when operating on both P-Mags, which went away when operating on either P-Mag alone. The roughness was felt through the floor, and other parts of the airframe structure. Advice from several posters, especially N941WR, helped focus efforts to a solution.
Yesterday I was able to get back to the hanger, and found that both P-Mags were timed 3 degrees before top center. I was also surprised to see that the A-curve jumpers had never been installed as intended. Re-timing to TDC and adding the A-curve jumper essentially retarded timing by eight degrees.
Two hours of test flight proved that the dual mag roughness was gone. So apparently the over advanced timing was creating roughness when operating on two mags, but when operating on a either mag alone the timing was retarded just enough to prevent the roughness. The roughness was subtle, but I suspect that it would have caused premature engine problems. Now I know what over-advanced ignition timing feels like.
Still not sure why / how the timing jumped by three degrees, both mags. My perception was that the roughness started abruptly on climb-out, but cannot be sure.
I intend to add an EICommander to the panel. This is a device that monitors the health of P-Mags during operation and allows additional manipulation of their performance.
Thanks to all who helped put the RV grin back on my face! And thanks to DR for keeping this great asset growing
- Roger
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11-15-2014, 04:59 PM
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Join Date: Jun 2012
Location: Frederick, MD
Posts: 821
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Thanks for the update
__________________
Dan Morris
Frederick, MD
PA28-140
Hph 304CZ
RV6 built and sold
N199EC RV6A flying
Learn the facts. "Democracy dies in darkness"
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11-15-2014, 08:05 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Hopefully the roughness you were feeling wasn't detonation.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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11-15-2014, 08:18 PM
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Join Date: Dec 2009
Location: Montreal
Posts: 1,456
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How can the timing change after 32 hrs? I understood that it had been running smoothly up to then?
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Scott Black
Old school simple VFR RV 4, O-320, wood prop, MGL iEfis Lite
VAF dues 2020
Instagram @sblack2154
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11-15-2014, 08:58 PM
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Join Date: Jan 2005
Location: SC
Posts: 12,887
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Great to hear the solution was that simple! I wish you many more trouble free hours! Don't hesitate to call, if you have any other questions.
Quote:
Originally Posted by sblack
How can the timing change after 32 hrs? I understood that it had been running smoothly up to then?
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It can't, unless the retaining nuts come loose and the P-mags turn but that didn't happen. They probably were not timed correctly from the beginning and it took him a few hours to recognize there was an issue.
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Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
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11-16-2014, 01:03 AM
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Join Date: May 2008
Location: Brisbane Qld. Aust.
Posts: 2,271
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Roger,
I am glad you fixed this up.
I would suggest setting the PMAG a degree or two past top dead centre, and on the less aggressive curve.

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David Brown
DYNON Authorised Dealer and Installer
The two best investments you can make, by any financial test, an EMS and APS!
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11-16-2014, 01:13 AM
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Join Date: Jan 2005
Location: SC
Posts: 12,887
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Quote:
Originally Posted by RV10inOz
Roger,
I am glad you fixed this up.
I would suggest setting the PMAG a degree or two past top dead centre, and on the less aggressive curve.

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As of firmware version 40, it is no longer nessisary to set them a degree or two past TDC. The reason is that that version now fires the plugs at four degrees past TDC for starting.
__________________
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
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11-16-2014, 03:44 AM
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Join Date: May 2008
Location: Brisbane Qld. Aust.
Posts: 2,271
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Thats a good thing, but I was meaning as an offset from the extra advance, a tiny bit helps, when LOP but they still have a bit too much otherwise in my opinion.
Each to their own. 
__________________
______________________________
David Brown
DYNON Authorised Dealer and Installer
The two best investments you can make, by any financial test, an EMS and APS!
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11-16-2014, 05:49 AM
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Join Date: Sep 2009
Location: Daytona Beach, Florida
Posts: 121
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Timing
When timing the engine get to TDC by turning the engine forward ONLY! If you turn it backwards the lash in the acc case gears will give you the result you had. Be safe!
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11-16-2014, 07:57 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by RKellogg
So apparently the over advanced timing was creating roughness when operating on two mags, but when operating on a either mag alone the timing was retarded just enough to prevent the roughness.
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This is typical. Dual plug setups require less advance than a single plug setup. In recip engines, it's all about timing the max pressure induced by the burning a/f mixture to occur just past TDC. Two ignitions from opposite sides of the cylinder will take less time to reach this max pressure point than one spark. There can be roughness when too much advance is in place, but surprised you could feel 3 degrees. I am still learning about lycomings.
Larry
Last edited by lr172 : 11-16-2014 at 08:02 AM.
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