912ULS rough running at idle
in 912 / 914 Technical Questions
3 weeks ago
The problem:
Rough running when on one ignition system (“A”) only, from idle through at least 4,000 RPM where I do the pre-flight mag check. The roughness is most notable at low RPM when I shut down from minimum idle. Additionally, I feel the engine is not as smooth as it has been during normal operation, I think what ever is causing this is somewhat noticeable at all RPM even with ignition selected to both. This all started about 20 hours of operation ago.
The aircraft:
A 2018, Sling 2 factory built S-LSA fitted with a 912ULS and Whirlwind ground adjustable prop.
2018 build engine - 210 hours total since new.
2018 build engine - 210 hours total since new.
5 year rubber exchange carried out 2 years ago.
Oil changes at 50 hour intervals, Shell Sport Plus 4.
Run exclusively on unleaded, ethanol free MOGAS, 95 RON. (Australia does not use “winter” blends)
Square type (later style) spark plug caps are fitted from new.
It has an ACS key type combination ignition/start switch marked “OFF R L BOTH START“ At this stage I am assuming “L” is system “A”. The wiring diagram is not detailed enough to confirm this and as yet, I have not physically checked.
It has coolant jacket type heaters on each carb, so “carb heat” is permanently on.
Run exclusively on unleaded, ethanol free MOGAS, 95 RON. (Australia does not use “winter” blends)
Square type (later style) spark plug caps are fitted from new.
It has an ACS key type combination ignition/start switch marked “OFF R L BOTH START“ At this stage I am assuming “L” is system “A”. The wiring diagram is not detailed enough to confirm this and as yet, I have not physically checked.
It has coolant jacket type heaters on each carb, so “carb heat” is permanently on.
Throttle action is smooth and light, does not creep.
Home airfield is 2,000’ AMSL.
Home airfield is 2,000’ AMSL.
Troubleshooting so far:
At 4,000 RPM, the mag check (RPM drop) is within tolerance on both systems with some mild roughness noticed on “A”. In fact, the RPM drop is very slight greater (worse ~20) on system “B” than the rough running system “A”. See numbers below.
RPM drop at idle is 90 RPM worse on ignition “A” with associated rough running.
The Carbs were IRAN as per the MM at 195 hours, no change.
At 4,000 RPM, the mag check (RPM drop) is within tolerance on both systems with some mild roughness noticed on “A”. In fact, the RPM drop is very slight greater (worse ~20) on system “B” than the rough running system “A”. See numbers below.
RPM drop at idle is 90 RPM worse on ignition “A” with associated rough running.
The Carbs were IRAN as per the MM at 195 hours, no change.
Idle jets set at 1.5 turns open. (I think it’s a bit rich at idle but I have no F/A ratio equipment to check).
Carbs balanced using a CarbMate device, no change.
New spark plugs made no difference.
Propeller dynamically balanced to 0.02 IPS. Blades are within 0.5 degrees pitch of each other. Tracking is not perfect but within limits according to Whirlwind and is not adjustable anyway.
Minimum idle is ~1,800 RPM but I maintain 2,000 during normal ground ops.
All cylinders are monitored for EGT with no obvious anomalies noted during normal ops or testing. There is no cold or cool cylinder, even when running rough on one ignition system.
Choke levers are confirmed operating correctly. They are closed when they are supposed to be.
The spark plug caps are very tight on the plugs.
The carb vent lines are secure and clear into the airbox.
Air filter is clean.
No fuel leaks noticed.
Fuel electric boost pump on or off makes no difference.
Fuel pressure is normal at all times.
Carbs balanced using a CarbMate device, no change.
New spark plugs made no difference.
Propeller dynamically balanced to 0.02 IPS. Blades are within 0.5 degrees pitch of each other. Tracking is not perfect but within limits according to Whirlwind and is not adjustable anyway.
Minimum idle is ~1,800 RPM but I maintain 2,000 during normal ground ops.
All cylinders are monitored for EGT with no obvious anomalies noted during normal ops or testing. There is no cold or cool cylinder, even when running rough on one ignition system.
Choke levers are confirmed operating correctly. They are closed when they are supposed to be.
The spark plug caps are very tight on the plugs.
The carb vent lines are secure and clear into the airbox.
Air filter is clean.
No fuel leaks noticed.
Fuel electric boost pump on or off makes no difference.
Fuel pressure is normal at all times.
Inflight mag check at 5200 RPM shows no noticeable increase in roughness and RPM drops equal at about 150 RPM.
I have followed the Lockwood Aero video guidelines for checking the ignition system https://www.rotax-owner.com/en/videos-topmenu/misc-videos/695-Rotax-lockwood-ignition-troubleshooting with the following results.
I have followed the Lockwood Aero video guidelines for checking the ignition system https://www.rotax-owner.com/en/videos-topmenu/misc-videos/695-Rotax-lockwood-ignition-troubleshooting with the following results.
Choke- no improvement when applied at any time during testing on either ignition system or both at idle or higher RPM.
Swap horizontal (trigger coil/module) connectors only- defect switched to the other system.
Swap vertical (module/ignition coil) connectors only- defect also switched to the other system.
4,000 RPM mag drop - “A” 90 - “B” 110. (Reversed when connectors swapped)
2,000 RPM mag drop - “A” 200 - “B” 110 (on “A” it’s rough at this RPM and again these reversed when connectors were swapped).
Can anyone recommend a course of action to further investigate this?
Many thanks in advance,
Des.
