Troubleshooting Ignition issues 912UL

“Then I swapped the horizontal connectors with each other (to my understanding, those are the ones that connect the coils to the modules) and did another runup. The 400rpm drop then switched sides, now showing the drop on the LEFT switch. To my knowledge, this means that the issue is downstream of the coils, meaning either the module or cables/boots.”

I’m not sure what you meant by “coils”, but the horizontal connectors connect the ignition modules to the trigger coils and charging coils. The vertical connectors connect the ignition modules to the double ignition coils. If the problem switched sides when you swapped the horizontal connectors, then you need to be looking upstream at the pickup/charging coils and wiring from them to the ignition modules.

Thank you for the clarification Jeff. Yes, in both cases it switched sides. The original problem happened when the key was in the RIGHT, and, after swapping the horizontal connectors it moved to the LEFT. Then I swapped it back to original and did the same with the Vertical connectors. The problem also moved to the LEFT. So regardless which connectors I swap (horizontal or vertical, not both at the same time), the problem switch sides. Is it fair to rule out problems with the modules and the ignition coils?

Thanks for clarifying.  The ignition switches are also wired via the horizontal connectors, So when you swap the horizontal connectors you would expect the problem to change to the other switch, not because the problem has switched to the other ignition module, but because the ignition on/off switches have been swapped to the opposite modules. So now switch A operates module B and vise versa.  The fact that it changed switches actually means the problem is not with the pickup/charging coils. Sorry my wording confused this earlier.

If you had the same effect when switching the vertical connectors, you are correct that the problem is downstream of the ignition modules as you suggest. I see you have replaced spark plugs and tested wires through the ignition coils, have you tested the primary side of the ignition coils (as per the MMH) and checked the condition of the connector pins at the vertical connectors on the coil side? 

I have checked visually the pins inside the connectors, everything looked good (to my relatively inexperienced eyes). I didn’t see any burns, disconnects or broken wires, for example. Maybe a loose or intermittent contact/pin perhaps?

About testing the primary side of the ignition coil, do you mean the boots or the trigger coils (back at the stator area)? I have not in either… the back of the engine is very hard to access in the Pipistrel, I would need to remove the radiator and part of the cooling circuit I think, so would want to do that only as a last resort :(

Another thing I am wondering is a possible loose/intermittent contact in one of the boots/cap… it would have shown in the resistance test I guess, but maybe it only happens with vibration? I could unscrew each one, check, and put it back on… as long as I don’t make it worse, lol. Is that the test you are referring to?

Do you think carburetor could cause something like that? I did a carb sync, optimizing for 2000rpm, before finding this problem. For some reason the carb synchronization doesn’t stay the same at all RPMs in my airplane, for example, it is super smooth and fully synched at 2000, but when running at higher RPM, it becomes “less” synchronized (like 2 leds in the Carbmate when at 4000rpm). Wondering if that could impact rpm during mag check.

I was referring to the primary side of the double ignition coils.  There is a resistance check procedure in the MMH.  Also, if you have any spark plug boots that don’t feel tight on the spark plug cable, just unscrew them, cut off a bit (maybe 10 mm) and screw the boot back on the wire.  Regarding the carb sync, yes this can be associated with a drop in RPM during the ignition check. However it’s more likely to show up as an out of spec RPM drop on both circuits. I’m sure there are circumstances where it would show up more on one side or the other, but I still think ignition is more likely.  But since this started after you did a carb sync, it’s something to check.  

It’s not uncommon to have a better sync at certain RPM than others. As a quick check you could sync again at 3500-4000 RPM and see how that affects your ignition check.  If this helps, then it would be good to start over with sync process, starting with the mechanical sync as outlined in the MML, and ensuring that both throttle cables are lubricated and move smoothly. Also, make sure both throttle cables are seated in the adjusters and safety wired so they stay seated.  The MML does not specify an RPM to sync at. I sync at 2000 and 4000 RPM, and if can’t get them both perfect, I split the difference.