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 » 2 weeks 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

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