weird mag behaviour

[prkaye]

My Slick mag just came back after being overhauled by aerosport (full engine IRAN). I think there is something going on with this mag when running at low RPM. But it seems to be fine at high RPM.

• In-flight mag check at 2400RPM, runs smoothly on the mag. 70 RPM drop.
• On both of the last two flights when i pulled the power back on turn to final, the PRM indication on my EFIS (which is fed from the p-lead on that mag only) momentarily dropped to 0 for a couple of seconds, before returning to normal RPM indication. This happened identically on the last two flights when i pulled the power back to about 800RPM for final (FP prop). This seems really weird??
• After landing i did another mag check at idle and it does not run smoothly on the mag at idle (RPM fluctuates).

I've checked wiring connections.
Thoughts?

 

[BobTurner]

I’d say the symptoms sound like spark plug misfire at low rpm, when the voltage out of the mag will be at it’s lowest.
1. Mag not putting out needed boltage due to ???
Or
2. Plug gaps too large. The P-mag can handle large plug gaps better than the mag, especially at low rpm. Did you interchange a Pmag plug with a regular mag plug?

 

[prkaye]

Thanks Bob. I installed 4 brnad new aviation plugs. If one plug was not firing correctly i would assume this would show up in an EGT number? Also, could a plug misfiring explain the momentary loss of RPM signal on the P-lead when i power back for landing?

 

[PaulvS]

I think to check the P lead connection to the EMS again because it sounds like the signal is not strong enough to be picked up. Even though the RPM on final is reading zero, the prop is still turning so that sounds like an electrical connectivity issue in the P-lead that may have been caused by re-fitting the mag after service.

 

[prkaye]

I checked the connections. But a loose p-lead connection to the EMS would not account for the symptom of rough-running only at low RPM, would it? I have two main symptoms:
1) rough running at idle when on just that mag
2) momentary drop-off in RPM indication (taken from the mag p-lead) when i transition from cruise to low-power on final
But mag performance at cruise RPM seems normal.
Is there a theory that would account for both symptoms?

 

[RV8JD]

I checked the connections. But a loose p-lead connection to the EMS would not account for the symptom of rough-running only at low RPM, would it? I have two main symptoms:
1) rough running at idle with fluctuating RPM
2) momentary drop-off in RPM reading when i transition from cruise to low-power on final.
But mag performance at cruise RPM seems normal.
Is there a theory that would account for both symptoms?

What is your idle RPM? If it is low, like 500 to 650, the engine will run rough.

 

[prkaye]

No it's about 700. And idle is smooth when running on the other ignition side, or on both. Just when i switch to only the left Mag at idle, it runs rough with fluctuating RPM (actaully fluctuating - you can hear it).

 

[PaulvS]

I checked the connections. But a loose p-lead connection to the EMS would not account for the symptom of rough-running only at low RPM, would it? I have two main symptoms:
1) rough running at idle with fluctuating RPM
2) momentary drop-off in RPM reading when i transition from cruise to low-power on final.
But mag performance at cruise RPM seems normal.
Is there a theory that would account for both symptoms?

Just guessing, with a bit of logic, that if the engine is running rough and the RPM signal is getting lost then a common denominator is the coil in the mag, which has just been overhauled. I would call Aerosport and describe the problem and see what they suggest.

 

[prkaye]

I made an interesting discovery today regarding that weird momentary loss of RPM indication. It happens only when i activate the flap motor! If I start lowering or raising the flaps, the RPM indication goes to 0, and then comes back when i stop operating the flaps. This happens in-flight and on the ground. The flap circuit is totally independent of the EFIS and the ignition, so i cannot understand how this is possible??

 

[Ralph Inkster]

I made an interesting discovery today regarding that weird momentary loss of RPM indication. It happens only when i activate the flap motor! If I start lowering or raising the flaps, the RPM indication goes to 0, and then comes back when i stop operating the flaps. This happens in-flight and on the ground. The flap circuit is totally independent of the EFIS and the ignition, so i cannot understand how this is possible??

Does your ignition magneto switch ground to a common chassis ground terminal (sharing the flap ground point), or a more direct path to the negative battery terminal? Its possible a poor ign (mag) switch ground is getting some feed back that's influencing your rpm sense wire.

Also for your mag rough idle performance, what is your leaning procedure on approach? full rich?

 

[tkracmer]

Check your engine to airframe ground.

 

[PaulvS]

If you may have an EFIS/EMS that displays the RPM and the flap position then check the connections, wire sizes and grounds for all the relevant items.

 

[lr172]

I made an interesting discovery today regarding that weird momentary loss of RPM indication. It happens only when i activate the flap motor! If I start lowering or raising the flaps, the RPM indication goes to 0, and then comes back when i stop operating the flaps. This happens in-flight and on the ground. The flap circuit is totally independent of the EFIS and the ignition, so i cannot understand how this is possible??

Reading RPM from the mag is challenging and no real surprise that external electrical noise is affecting it. The EIS is seeing a weak and somewhat erratic electron flow from the magnet through the coil and then it sees a break in it and counts this as 1/2 of a revolution. It is not at all like a tach signal. I mentioned in you other thread that mag drops at idle are going to be rough. You are not seeing it on the EI, as it is likely adavnced beyond 25 at idle.

It is possible that the shop messed up the egap in the mag and that would reduce the current flow nad make it harder for the EIS to see the signal. This would be most prevelant at low RPMs.

Did you by chance forget to put the ground lead from the P lead wire back on to the mag?

Larry

 

[prkaye]

Solved it! Last fall i had done the pre-wiring for a flap position sensor. I had wired from my GP input to the Dynon and put a terminal on it, but i hadn't taped the end. So I guess that GP input was grounded and that was causing some kind of interference. Yesterday i finished the flap sensor install and secured and heat-shrunk all the wires. On today's flight there were no anomolies with RPM readings when i activated the flaps
As for the rough idle on just the mag, i think my idle mixture may be too lean. I get no RPM rise at all when i pull the mixture to ICO. I'll have to try setting the idle mixture more rich.

 

[lr172]

I get no RPM rise at all when i pull the mixture to ICO. I'll have to try setting the idle mixture more rich.

It is easy to miss which is why I recommend the lowest MAP approach to setting idle mixture. I have posted how to do it a couple of times. Much easier and more accurate.