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  #1  
Old 04-10-2020, 07:26 AM
PandaCub PandaCub is offline
 
Join Date: Jul 2019
Location: Colorado Front Range
Posts: 25
Default My RV-14A doesn't like idling

Odd problem with the IO-390 Thunderbolt in my RV-14A: whenever I pull the throttle completely back, the engine seems like it's idling around 650 RPM but the tach goes haywire, reading anywhere from 30 to 1250. Engine has even stalled on me twice when I completely closed the throttle during taxi back to hangar.

Engine runs fine otherwise. One Slick magneto and one Emag. Issue can be seen in this short video:

https://youtu.be/Q7dfaJX1TUQ

Any thoughts appreciated.
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Colorado Front Range/Wyoming Big Horn Basin
RV-14A No. 140308 (N34PB) w/ 330+ Hours
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  #2  
Old 04-10-2020, 07:44 AM
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N941WR N941WR is offline
 
Join Date: Jan 2005
Location: SC
Posts: 12,887
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Are you picking up the tach signal from the P-mag? (Emag is the company.)

If you are, check the wire on pin 6 of the P-mag. Pin 6 is the tach signal, it could be loose.
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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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  #3  
Old 04-10-2020, 10:26 AM
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kbalch kbalch is offline
 
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Location: Clermont, FL
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I had exactly this issue during ground testing prior to the first flight. You’re likely picking up the tach signal from the P-mag, which does tend to go crazy when the RPMs fall too low. If your pin 6 was loose, you wouldn’t get a consistent signal even at higher RPM.

Try turning the idle adjuster in one full turn to result in a slightly higher idle RPM. Something around 750-800 should do nicely and will cure the issues you’re seeing.
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RV-8 N118KB (#81125) - Sold
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  #4  
Old 04-10-2020, 10:39 AM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by kbalch View Post
I had exactly this issue during ground testing prior to the first flight. You?re likely picking up the tach signal from the P-mag, which does tend to go crazy when the RPMs fall too low. If your pin 6 was loose, you wouldn?t get a consistent signal even at higher RPM.
...
What's "too low"? Mine will idle down to 560 RPM's and it is as steady as can be.
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RV-9 (Yes, it's a dragon tail)
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  #5  
Old 04-10-2020, 10:59 AM
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kbalch kbalch is offline
 
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Location: Clermont, FL
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Quote:
Originally Posted by N941WR View Post
What's "too low"? Mine will idle down to 560 RPM's and it is as steady as can be.
That's great, if pointless. It's also likely unachievable without dual P-mags. I can't think of a single thing achieved by an extra-low idle RPM. The engine only needs to idle low enough to minimize thrust with the throttle closed.

For my purposes, "too low" is the speed at which the engine stumbles, the tach indication is erratic, or both. For a variety of reasons, different installations will indicate a different RPM at this point. If one cares about the actual number, an optical Proptach may be used to confirm it.

An idle RPM of 750-800 is more than sufficiently low.
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RV-8 N118KB (#81125) - Sold
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  #6  
Old 04-10-2020, 11:20 AM
Bavafa Bavafa is offline
 
Join Date: Oct 2007
Location: Sacramento, CA
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Quote:
Originally Posted by kbalch View Post
That's great, if pointless. It's also likely unachievable without dual P-mags. I can't think of a single thing achieved by an extra-low idle RPM. The engine only needs to idle low enough to minimize thrust with the throttle closed.

For my purposes, "too low" is the speed at which the engine stumbles, the tach indication is erratic, or both. For a variety of reasons, different installations will indicate a different RPM at this point. If one cares about the actual number, an optical Proptach may be used to confirm it.

An idle RPM of 750-800 is more than sufficiently low.
Couldn't agree more and also to point out that Bill has a different engine and possibly a different prop, so that might idle easier.

However, my 390 + G3X + 2PMAGs did not show this behavior with the RPM even if I idled it at that low of speed. Mine is set to idle at around 900
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  #7  
Old 04-10-2020, 11:25 AM
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climberrn climberrn is offline
 
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Quote:
Originally Posted by kbalch View Post
That's great, if pointless. It's also likely unachievable without dual P-mags. I can't think of a single thing achieved by an extra-low idle RPM. The engine only needs to idle low enough to minimize thrust with the throttle closed.

For my purposes, "too low" is the speed at which the engine stumbles, the tach indication is erratic, or both. For a variety of reasons, different installations will indicate a different RPM at this point. If one cares about the actual number, an optical Proptach may be used to confirm it.

An idle RPM of 750-800 is more than sufficiently low.
My dual P-Mag idles around 600RPM and is rough at that speed. I was convinced to turn the idle up by another pilot. All was fine until I tried to stop on rollout. Used twice the runway. Returned the idle to the previous, lower setting. Now on the ground I can adjust the idle anytime with the little black knob! FYI, it doesn?t stall, just pretty rough at lowest set idle on ground. Perfect in the air.
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  #8  
Old 04-10-2020, 11:32 AM
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kbalch kbalch is offline
 
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Quote:
Originally Posted by climberrn View Post
My dual P-Mag idles around 600RPM and is rough at that speed. I was convinced to turn the idle up by another pilot. All was fine until I tried to stop on rollout. Used twice the runway. Returned the idle to the previous, lower setting. Now on the ground I can adjust the idle anytime with the little black knob! FYI, it doesn?t stall, just pretty rough at lowest set idle on ground. Perfect in the air.
What did you turn it up to? Also, the -14A is a heavier airplane than the -7A and, even if comparing identical idle RPMs, behavior on rollout at idle thrust will be different.

Apples and oranges...
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  #9  
Old 04-10-2020, 11:33 AM
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Walt Walt is offline
 
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Problem 1: adjust idle mixture and speed

Problem 2: erratic indication, has nothing to do with problem 1, this prob could be many things repair as nec.
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EXP Aircraft Services LLC
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RV7A built 2004, 2000+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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  #10  
Old 04-10-2020, 11:39 AM
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kbalch kbalch is offline
 
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Quote:
Originally Posted by Walt View Post
Problem 1: adjust idle mixture and speed

Problem 2: erratic indication, has nothing to do with problem 1, this prob could be many things repair as nec.
My experience (admittedly nowhere near as broad as yours) of several similarly equipped airplanes (one P-mag (generating the tach signal) and one Slick magneto) shows that Problem 2 is directly related to Problem 1. There may certainly be other possible causes for Problem 2, but stating categorically that it has nothing to do with Problem 1 is not consistent with observed reality.

When the RPM falls too low, the P-mag's tach signal, as indicated on the G3X, will fluctuate erratically over a wide range. Increasing the idle RPM will absolutely cure that problem.
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