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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.
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.
Throttle action is smooth and light, does not creep.
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.
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.
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.

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.
  • Re: 912ULS rough running at idle

    by » 8 hours ago


    Hi Des,

    Other than 4000 rpm test, I have never taken any notice of rpm drops below this engine speed and when I think about it, pretty much everything (air flow, ignition timing, rotational balance), is suboptimal - that the engine is not as smooth is to be expected.

    Given the inherent inaccuracies of most of our gauges, I can't see that a 20 rpm difference is anything to be concerned about.

    Years ago I had a rough running 912ULS - always seemed to commence when over Tiger Country. Spent many hours checking  systems, as you have. Could never find anything wrong. In the end decided it was psychological - problem went away & never reoccurred.

    Before going further - have someone else, with Rotax experince, do a check ride with you.

    😈.


  • Re: 912ULS rough running at idle

    by » 8 hours ago


    Thanks Sean, it’s not the RPM drop I’m concerned about. I’ve operated this aircraft from new, something has changed causing rough running.

    Cheers.


  • Re: 912ULS rough running at idle

    by » 6 hours ago


    No problemo Des!

    For carburettor balance, I use two identical Bowden Tube type gauges  ( eg see below). I only mention this because I have heard negative comments about the Carb Mate product  - no experince. Perhaps one of our Garus's will chime in.

    I don't think you mentioned checking the fuel system for contaminants?? (filters, float bowls, pump in casing filter).

    Fuel source - Have you tried going to another servo? Also - I use 98 RON,  95 or AvGas only if desperate.

    😈

     

    Motorcycle Carb Carburettor Synchronizer Vacuum Balancer Gauge 2


  • Re: 912ULS rough running at idle

    by » 6 hours ago


    Thanks again Sean, I think it is an ignition issue, as you can see in my initial post, the problem transfers from ignition A to B when I swap either of the connectors at the modules.

    I’m hoping someone on this forum has seen a similar issue that I believe I have described in detail.

    Cheers.


  • Re: 912ULS rough running at idle

    by » 3 hours ago


    "I think it is an ignition issue, as you can see in my initial post, the problem transfers from ignition A to B.........."

     

    To test your theory - anyone in your local, whom you could borrow a, known good, ignition from?😈


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