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  #1  
Old 06-29-2021, 02:55 PM
Flying Canuck Flying Canuck is offline
 
Join Date: Jan 2015
Location: Red Deer, Alberta, Canada
Posts: 571
Default Excessive mag drop

I've got 2 regular Bendix magnetos. I've been chasing down an issue with the help of my engine builder where the right mag is seeing drops of 300+ rpm pretty much every time I do a mag check. The plugs are all good, the coil is new. The mag runs perfectly on the bench.

My engine guy is checking the capacitor, in the meanwhile I was wondering if anyone might have any ideas. Could a faulty p-lead cause this? How about a connection to my engine monitor (p-lead pickoff with an inline resistor)?

For information sake, my left mag sees a drop of about 100. I check my mags at 1800 rpm. I have no issues while flying. My engine is an IO-320.

Thanks for any insights.
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Claude Pitre
RV-9A #91081, C-GCPT
Dynon SkyView HDX, IO-320 and WW 200RV C/S. Flying as of August 6, 2018

Added GPS 175 and authorized for IFR April 1, 2021

Interactive map of all of my flights here
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  #2  
Old 06-29-2021, 03:27 PM
BobTurner BobTurner is online now
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,769
Default

Have you checked the timing?
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  #3  
Old 06-29-2021, 04:19 PM
Taltruda Taltruda is offline
 
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 873
Default

What do you mean "Runs normally on the bench"? Do you have a machine that spins it up to rpm? How about sending it out to an accessory shop so they can perform a IRAN on it? When's the last time it was IRAN or overhauled by an accessory shop? There's a lot more to a mag than timing. .there's the points and capacitor, internal timing, dwell...
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Tom
Las Vegas
RV-4 flying…
RV-8 empenage finished 10-2020

Wings Started.. 11-2020
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  #4  
Old 06-29-2021, 05:01 PM
Flying Canuck Flying Canuck is offline
 
Join Date: Jan 2015
Location: Red Deer, Alberta, Canada
Posts: 571
Default

Quote:
Originally Posted by BobTurner
Have you checked the timing?
Yes, it's been checked and it's good.


Quote:
Originally Posted by Taltruda View Post
What do you mean "Runs normally on the bench"? Do you have a machine that spins it up to rpm? How about sending it out to an accessory shop so they can perform a IRAN on it? When's the last time it was IRAN or overhauled by an accessory shop? There's a lot more to a mag than timing. .there's the points and capacitor, internal timing, dwell...
To be honest, I don't know what I mean by that. It is my engine builder who is testing it, off of the plane. He rebuilds these things with some frequency and has 50 years of experience with them. While I can't quote his language in this forum, he tells me it sparks very well in his testing setup. He doesn't know what the issue is yet, but I'm fairly confident that if it is an internal mag issue he will find it. What I'm really wondering is if any external items would be able to cause this issue.
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Claude Pitre
RV-9A #91081, C-GCPT
Dynon SkyView HDX, IO-320 and WW 200RV C/S. Flying as of August 6, 2018

Added GPS 175 and authorized for IFR April 1, 2021

Interactive map of all of my flights here
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  #5  
Old 06-29-2021, 05:52 PM
00Dan 00Dan is offline
 
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 222
Default

Does the EGT indicate a plug not firing?

If not, is the engine running smooth despite the large RPM drop?
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  #6  
Old 06-29-2021, 06:34 PM
Flying Canuck Flying Canuck is offline
 
Join Date: Jan 2015
Location: Red Deer, Alberta, Canada
Posts: 571
Default

Quote:
Originally Posted by 00Dan View Post
Does the EGT indicate a plug not firing?

If not, is the engine running smooth despite the large RPM drop?
In an unfortunate coincidence, I had a failed EGT probe for the past while. I replaced it 10 days ago, and I haven't flown since (have confirmed that it works). As such I don't have reliable EGT information to look at. The temps on the 3 working probe were all similar, so nothing to suspect there. If there was a problem with #2 that was masked by the bad probe, I don't know. I did have the plugs checked and cleaned, replaced 4 spark plugs (also in the past 10 days). The mag drop continued after that work, although my left mag went from ~175-200 to ~100 with the new plugs. The Champion plugs that were replaced failed testing so we put in new Tempest plugs (the other 4 are also Tempest).

Engine is running smoothly in the air. Warm ground running is rough, but I can ease that with aggressive leaning.
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Claude Pitre
RV-9A #91081, C-GCPT
Dynon SkyView HDX, IO-320 and WW 200RV C/S. Flying as of August 6, 2018

Added GPS 175 and authorized for IFR April 1, 2021

Interactive map of all of my flights here
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  #7  
Old 06-29-2021, 06:39 PM
bill.hutchison bill.hutchison is offline
 
Join Date: May 2020
Location: Northern VA
Posts: 298
Default

I had this same symptom and problem once on a PA-28 I flew in a club.

After rebuilding the mag twice, they finally diagnosed it correctly.

They told me it was bad wires.
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  #8  
Old 06-29-2021, 07:09 PM
BobTurner BobTurner is online now
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,769
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Lycoming SB 1132B says you should lean mixture for max rpm, reduce throttle back to mag check rpm, check again. If within specs (max 175 rpm drop, two mags within 50 rpm) then all is well.
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  #9  
Old 06-29-2021, 07:48 PM
Flying Canuck Flying Canuck is offline
 
Join Date: Jan 2015
Location: Red Deer, Alberta, Canada
Posts: 571
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Quote:
Originally Posted by BobTurner View Post
Lycoming SB 1132B says you should lean mixture for max rpm, reduce throttle back to mag check rpm, check again. If within specs (max 175 rpm drop, two mags within 50 rpm) then all is well.
Not possible to lean for rpm with my C/S propeller (at least I don't know how). Mixture doesn't seem to affect this particular issue. If the engine is warm, I need to lean aggressively for smoothness. I don't tend to mag check when full rich. My left mag is generally 175 or less, right mag is at least 300.

I should add, I have separate mag toggle switches, when I am checking the right mag, that is to say the right switch is on and I'm cycling the left switch.
__________________
Claude Pitre
RV-9A #91081, C-GCPT
Dynon SkyView HDX, IO-320 and WW 200RV C/S. Flying as of August 6, 2018

Added GPS 175 and authorized for IFR April 1, 2021

Interactive map of all of my flights here
Reply With Quote
  #10  
Old 06-29-2021, 08:02 PM
BobTurner BobTurner is online now
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,769
Default

Quote:
Originally Posted by Flying Canuck View Post
Not possible to lean for rpm with my C/S propeller (at least I don't know how)
With prop control full forward the prop should be against the fine pitch stop, until and if you get to RPM red line. Increasing power from 1800 rpm (whether the throttle, or setting best power mixture) should increase rpm.
Set 1800 rpm.
Lean mixture to maximize rpm.
Reduce throttle back to 1800 rpm.
Re-do mag check.
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