912is rpm drop

Hi Jeff,

Sorry for that.
I didn´t really explain it because it is so very similar to what Marty explained.

We have a severe RPM drop when turning off Lane A. We found one of the coils to be measuring faulty and replaced it. The problem seemed to be solved, but after a quick test-flight it was back again.

We tried all of the nice tips you provided Marty in this thread as I described above, but wasn´t able to find the fault.

Fredrick,
Seems you have a problem on Lane B, are you getting a fault light on lane B? When this happens, do you see one or more cylinders with a change in EGT?  If you see just one cylinder with an EGT change, then it’s more likely it could be an injector problem on that cylinder. It would of course be the injector on lane B. Typically a failed injector would give a lane fault, but a clogged or partially clogged injector may not.  But with a clogged injector on the B lane, you should see a cold cylinder when you turn off lane A. With this only showing up during a lane check, I would not suspect a compression/mechanical problem.  This is either fuel or spark.  Good luck and please report back with what you find.  

Thanks for the input!
No, we do not get a Lane B light.
I agree it is probably either Spark or Fuel. We started looking at the injectors and disassembled and cleaned them. They all worked fine and showed no sign of clogging.
There is a very significant drop in rpm and the engine is running extremely rough when we turn off Lane A, so we have not let it run for long enough to properly look at temperature drops.

Right now we are thinking of disassembling the wings and driving it on a trailer 7 hours to our closest Rotax shop

Fredrik,

Pulling the wings and a seven hour trip sounds like a pain.

Have you done all the basics to check lane B? Lane B uses coils 2 & 4, and all the top spark plugs.  Have you checked coil resistance, spark plug wire resistances, spark plug boot resistances and the spark plugs for Lane B?  You said you changed one coil, I assume that was 2 or 4.  You could swap the other Lane B coil with one of the Lane A coils (1 or 3) and see if the problem changes lanes.  

I remember you said after changing a coil the problem seemed resolved at first.  So it seems like you might have touched something that made a temporary difference.  Were the connectors tight on the primary side of the coil? There were some problems with the earlier primary side terminals getting loose, and Rotax’s changed connector types a couple years ago.  It’s perplexing that your problem is not causing a lane fault.

Fredrik,

I know the pain you're feeling. I believe I am at that point now also. 7 hrs to the nearest place also on a trailer. I have replaced coils and inspected plugs and am not finding anything obvious. Just frustrating is all.  3000 hours general aviation on Lycoming and Continental and not one flight canceled and after a year of this Rotax I have about 7 hrs total. Most of those hours are flying trying to figure out what the problem is.