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My RV-14A doesn't like idling

PandaCub

Active Member
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.
 
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.
 
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.
 
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.
 
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.
 
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
 
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.
 
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... :)
 
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.
 
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.
 
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.
Exactly why I have my idle turned down as low as I do.

The -9 with a FP prop is a glider and at "normal" idle speeds, it is a real challenge getting it to slow down on final.
 
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 point was that the P-mags should put a reliable tach signal at low RPM's and if they don't, time to contact Emag.
 
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.

My point is how an engine idles, has nothing do with tach indication unless the erratic tach is caused by the pmag failing.

To put it another way, I don't adjust idle rpm to fix an erratic tach.
 
My point is how an engine idles, has nothing do with tach indication unless the erratic tach is caused by the pmag failing.

To put it another way, I don't adjust idle rpm to fix an erratic tach.

+1

it is also likely that the erratic tach is causing the rough idle, given the Pmag also uses the RPM data to set timing. It is probably rapidly oscillating from 0 BTDC (likely for RPMs below 3-400) to 30 BTDC in timing due to the unreliable tach signal.

Larry
 
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Have you adjusted the idle mixture? Are you getting a slight rise (50 or less) in rpm just prior to idle shutoff as you are leaning?
 
An idle RPM of 750-800 is more than sufficiently low.

Not in all cases. On my 6 with an FP prop, I am able to slow much better on final with a lower than desired ground based, idle speed setting. This setting that helps with slowing down on final leaves my ground idle around 650 or less and I have to use the black knob to get it back to 750-800 where the engine is happy. I have read numerous posts where others do the same for the same reason.

This is not the case on my 10 with a CS prop.

Larry
 
+1

it is also likely that the erratic tach is causing the rough idle, given the Pmag also uses the RPM data to set timing. It is probably rapidly oscillating from 0 BTDC (likely for RPMs below 3-400) to 30 BTDC in timing due to the unreliable tach signal.

Larry

The P-mags don't work like that.

Below 400 RPM's they set the timing to 4.2° after TDC.

Between 400 and 1000 RPM with low MAP they set the timing closer to TDC.

I have mine set to 25.2° for takeoff and the P-mags have moved the timing from 16.8 up to 25.2° in 1.4° increments untill I apply full throttle. (The P-mags may move the timing closer to TDC at lower ROM's, I just haven't seen it.)

This is done that way to help with low RPM idle speeds.

Based on the video, it didn't sound like the engine was dieing, just the tach signal was dropping out.

I would recommend the OP contact Emag.
 
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Thanks to all for the replies and discussion. I should also mention that I see about a 150 RPM drop between the Slick magneto and the E-Mag unit during runup. Not sure if that's proper (or even related) but I've always wondered about it.

Also, at the outset my A&P did send the E-Mag unit back to the factory for a rebuild. They said it had "corrosion" inside that could have been caused by someone washing the engine, and yet, even after rebuild, the tach is still doing the same thing (i.e. the video I posted is AFTER E-Mag rebuild).

A head-scratcher, for sure...
 
Thanks to all for the replies and discussion. I should also mention that I see about a 150 RPM drop between the Slick magneto and the E-Mag unit during runup. Not sure if that's proper (or even related) but I've always wondered about it.

Also, at the outset my A&P did send the E-Mag unit back to the factory for a rebuild. They said it had "corrosion" inside that could have been caused by someone washing the engine, and yet, even after rebuild, the tach is still doing the same thing (i.e. the video I posted is AFTER E-Mag rebuild).

A head-scratcher, for sure...
It is common to have an RPM drop when grounding the P-mag and running off the slick. However, there should be no noticeable difference when grounding the Slick and running off the P-mag.

I'm not worried about the corrosion in the P-mag as I'm sure Emag would have repaired any damage. Especially since they noted it.

What is a concern is that your mechanic removed it. I would look at the wire and connection to make sure there is no corrosion there.

Two other questions, had you modified the internal configuration on the P-mag with our EICommander or Emag's EICAD program? If so, it would have been reset to factory defaults when you returned the P-mag. My other concern is, does your mechanic have experience with P-mags and did he time it correctly? These get timed in a way that is significantly different than a Slick mag and a number of mechanics have not done it correctly.
 
Exactly why I have my idle turned down as low as I do.

The -9 with a FP prop is a glider and at "normal" idle speeds, it is a real challenge getting it to slow down on final.

Careful about posting about another plane besides a -14 over here. Another poster said comparing a 7 and 14 idle speeds is like comparing apples to oranges. We both have injected, 4 cylinder lycomings with CS props. I?m sure the -14 is way different. Whatever.

Back to the original question. Looked at idle speed today, 450-500 RPM. Rough idle at those rpm?s but I never keep it set there. The Dynon reading was jumping around a little bit, 50 RPM or so. Am very happy with performance at that idle.

Do you have the RPM pickups on both the PMag and slick? Does the slick have a resistor in line? Dynon calls for one but not sure if Garmin calls for one. You could disconnect either the mag and see if the problem is isolated. The sound from the engine in the video is consistent.
 
I am with Walt. Especially step 1 of his post. Instead of fixating on the idle speed, adjust your idle mixture for proper idle cut off RPM roll. Then set your idle speed. You will find as you optimize the idle mixture setting the idle will automatically change up or down. It's kind of an iterative process adjusting each alternatively but if the idle mixture out of kilter it will cause rough running engine on the ground. The idle mixture is usually a set it and forget it for hundreds of hours but become self evident every time the engine is killed by observing the RPM rise. The idle speed setting is secondary and might need adjusting more often.
 
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Not a single thing achieved by low idle.

Float plane or amphib.
The lower your RPM, the more time to jump out and dock not risking ramming your floats into some riverbank or rocks. (SuperCub O-360 carb, slicks, 470 idle RPM.) Granted only one RV on floats I believe :)

KBALCH: "That's great, if pointless. It's also likely un-achievable 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."

'
 
Careful about posting about another plane besides a -14 over here. Another poster said comparing a 7 and 14 idle speeds is like comparing apples to oranges. We both have injected, 4 cylinder lycomings with CS props. I?m sure the -14 is way different. Whatever.

Back to the original question. Looked at idle speed today, 450-500 RPM. Rough idle at those rpm?s but I never keep it set there. The Dynon reading was jumping around a little bit, 50 RPM or so. Am very happy with performance at that idle.

Do you have the RPM pickups on both the PMag and slick? Does the slick have a resistor in line? Dynon calls for one but not sure if Garmin calls for one. You could disconnect either the mag and see if the problem is isolated. The sound from the engine in the video is consistent.
The point was, as stated above, the P-mags will work at very low idle speeds.
Again, if the P-mag isn't putting out a stable tach signal even at very low RPM's, something is wrong with the P-mag.
 
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