Hello, after 3 months of hangar, I tried to restart my 914UL equipped Europa XS. I found that the mag drop at run-up was lower than usual (at least that I remember). My engine has 350 hours with fully renewed carbs last Aug and 4 pins black boxes. At 4000RPM, I got a drop of 300RPM on A and a drop of 400RPM on B. I always had a slightly lower drop on one mag than the other one. But this time, I found it a bit too much and the engine was clearly running rough on mag B.

So I already went through the following items:
- sparks plugs have been changed about 50 hours ago, but changing these is next on my list.
Checking these, I found that the bottom spark plugs on cylinder 1 is having the color depicted under:
http://www.rotax-owner.com/rotax-blog/item/7-the-basics-on-my-spark-plugs -> This plug is oiled fouled, caused by poor oil control.
The top one also has a strange color but more velvet than black
So I really suspect that my issue is located with cylinder 1
- swapped the position of the charging coils (red cable coming from the back of the engine to the black boxes), same mag is dropping, so no issue there
- swapped the pick-up coils connectors (4 pins). Then, instead of having a drop of 300/400 at 4000RPM, I got a drop of 200/600. But on the same map, so seems that the pick-up coils are OK. But the difference in drop is extremely strange...
- check the resistor between top 1/2, top 3/4, bottom 1/2 and bottom 3/4 spark connectors and I got everything between 15.5kohm and 16.7kohms. So seems no issue here.
I well saw that some of the spark plugs connectors were not always giving perfect contact with the multimeter pins.

Next steps means removing the black boxes. But I first wanted to change the spar plugs before disassembling too much.

And the fact that I got totally different drops when swapping the pick-up coils to the black boxes gave me the idea that potentially the original pick-up coils connections to the black boxes could be wrong. The builder and first owner of the aircraft had some issues with ignition and I wouldn't be surprised that he switched the connectors of these pick up coils to the black boxes.
I was thinking of checking all connections from the stator to the spark plugs but that will mean removing the black boxes. And before I do that, I wanted to hear your thoughts on this issue.

I was also thinking of getting a stroboscope to make the measurements of sections 3.9 and 3.10 of the heavy maintenance manual, but I have to admit that I don't really understand how to practically measure the cut-in speed and the transition from start to operation timing.

Thanks, Jacques.
  • Re: 914UL mag drop

    by » 11 years ago

    Hi Jacques,
    It is unlikely you have an ignition problem unless you are correct about the prev owner messing with it but then it would have had this problem before?
    More likely is that your turbo check valve has weeped during the storage period and you now have oil blown into the airbox/carbs/cylinders. Do you have oil dripping out of the airbox vents? If so remove the turbo outlet hose and carefully clean out the oil. You may need to clean out the airbox if it is really filled up.
    The other most likely cause is carbs; it could be too lean, is it colder than normal in your area? If so just put the needles one notch richer. I did this to a 914-3 yesterday and went from a 450rpm mag drop to 200rpm.
    Please note the 914 ignition is staggered so it is common to have a difference between A and B: the A box fires all top plugs and the B box fires all the bottoms. Rotax does this so the B can be fired at 22 BTDC and the A fires at 26 BTDC to give a nice even flame front thereby avoiding detonation.
    Hope this helps, normally after chasing the ignition for a few days it turns out to be carbs/induction.

  • Re: 914UL mag drop

    by » 11 years ago

    I have experienced occasions where the mag drop changes with prop load. A VP prop set to fine may give a big mag drop, but when set to course the mag drop is almost unnoticeable. I believe this is result of the mixture distribution in the cylinder & the 914 ignition configuration -A Top set, B Bottom set of plugs.

  • Re: 914UL mag drop

    by » 11 years ago

    Rob, indeed I found oil in the hoses between the turbo and intercooler, intercooler itself and between the intercooler and the airbox. Some of the the spark plugs were just black. I cleaned up all that. The airbox seems to have very limited amount of oil left, so I will try like that during next week-end. Anyway, the engine was already running way smoother after getting new spark plugs and having run for 1.5 hours.

    Many thanks for the "to the point" advice.


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