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Greetings,

I've been troubleshooting (unsuccessfully so far) a problem with my A ignition.  Near idle, both A and B seem fine, but at 3500 where I do my runup (brakes won't hold 4000), BOTH seems fine, until you go to the B ign.  B is smooth, and A is rough.  It's quite noticeable, but not like you're missing a cylinder or anything.   When you go back to BOTH you can tell that BOTH isn't as smooth as B.  I feel like this has been slowly getting worse over time, and it's only recently started to concern me.   The RPM drop during the ignition test is only about 130-150, with the lower drop being the rough A ignition which makes no sense.   

1- Replaced the plugs, no change

2- Swapped ignition modules, no change.  Still A that's rough. 

3- Ohmed across wires and coils (1T-2T, 1B-2B, etc).  The plug connectors were dirty enough that it wasn't a great test, but 1T-2T seemed higher. 

4- Replaced all plug resistor caps, cutting off 1/4" of cable on each.  This made the resistance test reliable, 15k on all but 1T-2T, which was 17.5k.  I removed the caps for 1T and 2T, and verified that the extra resistance is not the new plug caps, so it must be the wires or coil.  No change in roughness, and now I have some radio noise as well.  Awesome...

I did check the pickup coils to make sure they weren't loose, but I haven't checked the resistance yet, or the gap.  I'd be surprised if it was either of those. 

At this point, my primary suspect is the coil that runs 1T-3T.  I need to verify that it's got a solid ground, though that wouldn't cause the resistance across the outputs to go up.  Getting to the wires on the coil is of course a pain. Is it possible to measure each output coil separately?  In other words, instead of measuring from 1T to 2T, would you get a reading from 1T to ground, and 3T to ground?  If so, I'd expect that to be half the normal coil resistance for each side.   

For the next test, I'm thinking I can swap the input connectors for the coils between modules, of course staying with the same cylinder (1T-2T with 1B-2B, and 3T-4T with 3B-4B) I'm thinking that would be relatively easy, and safe.  If A continues to be rough, then the problem is the trigger coils or wires before the module, and if it moves to B then it's down to coils or wires.   Is this a known good troubleshooting test?  I can't see any reason why it wouldn't work, but I figured it might be safer to ask. 

How often do the coils go bad?  Anyone in the US got old style coils laying around?  I have another ancient engine on a test stand that I can probably borrow parts from if needed, though I hate to break them both at the same time :-)

Thanks,

Rusty

 

  

 

  • Re: A ignition rough, possible coil issue

    by » 3 months ago


    How many hours since both carbs were rebuilt?  Had a similar problem last year and, although not intuitively obvious, the carbs were the problem, not the ignition system.  Once the carbs were done, all was well again...

    Suggest also looking at this video on the Rotax-owner.com web site.

    ROTAX Tech Talk: Troubleshooting the Rotax 912/914 ignition system


  • Re: A ignition rough, possible coil issue

    by » 3 months ago


    The carbs were rebuilt maybe 3 years ago, just before I got the engine.  It was done at one of the service centers.  I certainly can't think of a way this could be carb related, but never say never. 

    I hadn't noticed the video, so I'll check that out.  Thanks.

    Rusty

     


  • Re: A ignition rough, possible coil issue

    by » 3 months ago


    Try shutting off one ignition at a time in the air at cruise RPM.  I'm betting both ignitions will run smooth with very little drop in RPM.  

    I do this test on almost every flight and for sure before flying over water....

    If above test is good, then carbs are out of synch in mid-range.  Synch carbs at off-idle and live with mid-range roughness.  It is necessary to have good synch at off-idle so L-R power pulses are evenly felt at the gearbox.  Gearbox can tolerate mismatch at higher RPM, but detrimental if mismatch at low RPM.  Cruise RPM should show fairly close match L-R EGT.

     


  • Re: A ignition rough, possible coil issue

    by » 3 months ago


    I was going to ask about shutting them off in flight.  I remember some ignition I had previously that specifically said not to shut them off at higher RPM, but I was 90% sure that was some other engine, and not the Rotax.  I thought trying it on the ground might be safer, but it's good to know trying it in flight is OK. 

    I watched the video, and picked up a couple ideas.  Interesting to note that he said roughness in the 3500 range was almost always the carbs.  I know idle RPM is OK with both ignitions, so maybe higher RPM will be too. I'll still be surprised if this is carb related.

    I hadn't thought about swapping the 4 pin connectors (old modules).  From the schematics, that seems harmless, and would tell me the same thing I was proposing to do with the coil wires.  This will be much easier, and less confusing.   

    Thanks,

    Rusty

     

     


  • Re: A ignition rough, possible coil issue

    by » 3 months ago


    Our AM aircraft radios are an excellent tool for listening to ignition problems. Just tune your radio to a free frequency and turn the squelch off so that all of the static is audible. Now listen to your on ground ignition system checks. Be aware that you will also be able to hear strobes, electric fuel pumps, usb connections and all manner of other electrical interference as well. Its a bit like a doctors stethoscope for aircraft!

    If I remember correctly that 1/4" lead cut off that you have done already at the plug cap end can be done at the coil end as well? Just make sure that you clean the leads of dust and salt before reinsertion.

     

    Regards........Chook.


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