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  #11  
Old 11-24-2020, 06:49 PM
MartinPred's Avatar
MartinPred MartinPred is offline
 
Join Date: Nov 2005
Location: Overseas
Posts: 157
Default Separate Mag Switches

I forgot to mention that I don't use a key switch. I have separate toggle switches for the left of right mags, with the starter on the stick. This allows me to keep good control of the stick and throttle when starting.

So I can't see how it's a bad switch. But I suppose the hot P-lead trick is worth a shot.

The full ground check looks something like this:

Both on - smooth engine running.

Left off, Right on - ~100 rpm drop

Left on, right off - no change

Left off, right off - engine dies

right off, left off - engine dies

Impulse coupling is on the left mag. I always start with the left on, right off.

I'll grab my local A&P buddy to help me double check the mag timing. I'm using a standard timing light and both the red and green lights with the tone come on and off at the same time through full rotation. I'll also double check the timing stripe. Maybe I have the right one 180 degrees off?

-Matt
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  #12  
Old 11-24-2020, 07:47 PM
Taltruda Taltruda is offline
 
Join Date: Oct 2017
Location: Las Vegas, NV
Posts: 524
Default

Quote:
Originally Posted by MartinPred View Post
I forgot to mention that I don't use a key switch. I have separate toggle switches for the left of right mags, with the starter on the stick. This allows me to keep good control of the stick and throttle when starting.

So I can't see how it's a bad switch. But I suppose the hot P-lead trick is worth a shot.

The full ground check looks something like this:

Both on - smooth engine running.

Left off, Right on - ~100 rpm drop

Left on, right off - no change

Left off, right off - engine dies

right off, left off - engine dies

Impulse coupling is on the left mag. I always start with the left on, right off.

I'll grab my local A&P buddy to help me double check the mag timing. I'm using a standard timing light and both the red and green lights with the tone come on and off at the same time through full rotation. I'll also double check the timing stripe. Maybe I have the right one 180 degrees off?

-Matt
Bingo. .you can't time mags going through a full rotation. . The left mag needs to snap the impulse coupling, then you back up the prop a few degrees, and go forward again (to get the gear slack out) timing the mag after the impulse coupling.
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  #13  
Old 11-24-2020, 08:29 PM
Flying8 Flying8 is offline
 
Join Date: Jun 2020
Location: Mayport FL
Posts: 15
Default

Darn! Yeah separate switches makes it a little more difficult. I was hoping it was a simple key switch.
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RV-8 tail complete 11/3/20
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  #14  
Old 11-24-2020, 09:49 PM
gasman gasman is offline
 
Join Date: Mar 2007
Location: Sonoma County
Posts: 3,874
Default

Yep! I also expect left mag NOT timed properly.
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  #15  
Old 11-25-2020, 06:12 AM
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MartinPred MartinPred is offline
 
Join Date: Nov 2005
Location: Overseas
Posts: 157
Default Timing

Wow. Ok. I will give that a try.

-Matt
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  #16  
Old 11-25-2020, 09:33 PM
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MartinPred MartinPred is offline
 
Join Date: Nov 2005
Location: Overseas
Posts: 157
Default All in the Timing

Yep. That was it. Turns out I had the impulse coupling timed to 25 degrees before TDC. Which means the left mag was actually 50 degrees before TDC (!). Itís amazing it ran at all!

Did an engine run and a test flight with the correct timing set. Itís like a new plane! High CHTs on the right side disappeared. And tons of power.

Thanks everyone for the help!

-Matt
N402BD
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  #17  
Old 11-26-2020, 02:00 AM
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rv8ch rv8ch is offline
 
Join Date: Feb 2005
Location: LSGY
Posts: 3,444
Default

Quote:
Originally Posted by MartinPred View Post
Yep. That was it. Turns out I had the impulse coupling timed to 25 degrees before TDC. Which means the left mag was actually 50 degrees before TDC (!). Itís amazing it ran at all!

Did an engine run and a test flight with the correct timing set. Itís like a new plane! High CHTs on the right side disappeared. And tons of power.

Thanks everyone for the help!

-Matt
N402BD
Matt, thanks for closing the loop on this one. I can guess that the first spark came in so early it didn't have a chance to burn much of the fuel/air mixture before it was compressed, but will leave a good explanation to more experienced folks.
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  #18  
Old 11-26-2020, 07:09 PM
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rjcthree rjcthree is offline
 
Join Date: Jan 2007
Location: Bay Village, OH
Posts: 915
Default I feel bad....

Us dual mag folks probably should have seen this one. We let the impulse release before working the prop Backwards to check timing...glad you solved it. I admit to having the same issue...and can tell you the Lyc will run with HUGE advance at low rpm, pretty well, thank you.
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RV-9A N183RC/90432: tip-up, O320H2, Ellison, Dynon D180, CPi2, Sen GA, at KLPR
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2011-2021 VAF contributor
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