I would appreciate any advise on my current issue with my 2007 Rotax 912UL 80hp engine.  I changed my spark plugs during my recent compliance inspection.  The old plugs other than #3 cyl. were sooty/black.  #3 cyl plug was much cleaner and appears to be running lean.  

I installed new plugs and removed the top new ones after about 4 hour use. The spark plugs in Cylinders 1,2 & 4 were black/sooty but after cleaning them the insulator was a light chocolate color.  Cylinder #3 was a bit sooty but the insulator was white before cleaning it.  Could #3 Cyl. be running lean?  I checked the Rt. Carb. floats to cyl. 1 & 3 and they weighed ~ 3 gram each.  I also checked the left carb. floats (cylinders 2&4) and they were ~6 gram each, well over the <7g total allowed.

 So new floats will be installed on both carbs but I still don't have a fix for the #3 cylinder running a bit lean.  Only thing I can come up with is an air leak in the #3 cyl. intake manifold to head O-ring causing #3 to run lean.

 Any help will be very much appreciated.  As a side note all cylinder compression's were 79/80.



  • Re: 912 UL Spark Plug concern

    by » one year ago

    If you were idling too long then you can get that DRY black soot, but when you fly that burns off at the higher rpms. First make sure the plugs are gapped correctly. Too wide a gap can add to your issue. Clean the plugs and run the engine up around 3500+ rpm for 5 minutes then immediately shut it down. No idle time or go fly and when you land shut the engine down and no long taxi's or sitting at idle. After either of these then check the plugs. I doubt they will be that dry black soot. 

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: William Campbell

  • Re: 912 UL Spark Plug concern

    by » 6 months ago

    Hi William,

    What was the final conclusion or resolution?

  • Re: 912 UL Spark Plug concern

    by » 6 months ago

    Hi Chari,

    I replaced the plugs and it has been running great.  One rule I have is if it is running great I don't mess with it.  The ignition checks are smooth and within the recommended drop.  I will check the plugs if I notice something in the ignition check or in the general overall performance of the engine or at the next compliance inspection.

    Have you had the same problem?


    Bill Campbell

  • Re: 912 UL Spark Plug concern

    by » 6 months ago

    Just to confirm - you just put the plugs back in and continued as the engine runs fine?


    yes, my #3 tends to run slightly leaner than the other 3 based on how the plugs look.

    My main concern however is a severe spike in EGT on this cylinder when I reduce power from WOT (26” MAP +) to around 23-24”. I have a constant speed prop.

    EGT goes from 815C to 940C.

    I have tried everything to eliminate an instrument error. Also checked for an induction leak. 
    Currently running carb needle clip on richest setting.  Rear plugs light brown, but engine is not happy in the 2000-2300rpm range.


  • Re: 912 UL Spark Plug concern

    by » 6 months ago

    Hi Chari,

    I was not very clear about the spark plugs.  I replaced all 8 with new NGK spark plugs.

    Also on the idle low RPM roughness.  There is a ground wire from each ignition that grounds to the intake manifold.  On my 912UL the ground wires are bolted to the intake manifold between cylinders #1 & 3.  They may be a little difficult to see and remove but take them loose and clean up the ends of the wires and make sure the manifold is clean around where the bolts go in.  Use some dielectric grease on the wire ends and reassemble.

    I had a problem where my engine was rough at idle and no one could correct the problem.  Carbs rebuilt, Carbs balanced and nothing would smooth it out.  Then a friend told me to clean the ends of the ground wires to the ignitions.  When I removed them from the intake manifold the wires were corroded so bad that a good connection could not be made.  After reassembling I started the engine and it was amazingly smooth at idle and beyond.  That was a few years ago and it still idles great today.  So lesson learned - some of the simplest things can cause a big problem.  Prior the finding this ground problem we were trying to put bandages on the problem and not finding the root cause.

    Please let me know how things work out for you.


    Thank you said by: Rotax Wizard, RotaxOwner Admin, Peter Kl.

You do not have permissions to reply to this topic.