Hello all,

So I noticed the last couple flights I had uneven CHTs on my 912 ULS.  So today I started to investigate.  

First I swapped the temp sensors, and the problem stayed the same.  Number 1 low and number 2 high (See pics)

So I then went and pulled plugs and found one sooted up.  Sadly today, I only pulled the top plug, should have pulled the bottom, but will tomorrow.  

The plug that is all sooted up comes from the front right (looking from the cockpit, closest to the prop).  The sensor on this side is on the read cylinder.

All the other top plugs are a nice colour.  If it were carb balance, wouldn't both cylinders be affected?

Any ideas?

8750_1_IMG_1363.jpg (You do not have access to download this file.)
8750_1_79ea8831-d0ca-4a34-b976-36490079bfc5.JPG (You do not have access to download this file.)
  • Re: Uneven CHT and investigation

    by » one year ago

    Some further information.  

    I pulled the bottom plug and also found it black. 

    I did a full leak down test and all checked out.  

    next step is carb balance, but I am still not convinced that will cause uneven CHTs. 

    Any assistance would be very helpful. 



  • Re: Uneven CHT and investigation

    by » one year ago

    You have 2 CHTs and 2 EGTs.
    Which 2 are they?  Are they the same 2?
    Don't assume! Trace them out.
    The EGTs are Welded in the Exhaust headers about 2 inches from the heads.
    The CHTs are bolted into the heads.
    The unused CHTs have Plug screws in the unused holes.

    If the CHT are #1 and #2; remember that #1 is on the Right and #2 is on the Left.  Different Carbs!
    It is not unusual to Monitor #1 and #4.  Front Right and Rear Left.
    #4 will naturally be "Slightly" hotter because the #2 cylinder in front of it gets the cool air first and #4 gets the warmed leftovers.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

  • Re: Uneven CHT and investigation

    by » one year ago

    Update - SOLVED. 

    So sometimes you can’t see the forest for the trees. The problem was staring me in the face and I could not see it. 

    I decided to just go back to basics and look at what in the system can cause a mixture issue. The very first thing that came to mind was the choke. So I tested between full choke and no choke and found that the choke was binding and not returning to min.  

    It would appear that the Lame that did the last 100 did not lubricate the throttle components.  I also found the idle mix screws to be way out.  Once I fixed that and lubricated all linkages, the problem is all but gone. The CHT are within 10c of each other. 

    I have now synced carbs and once the weather clears will test fly. 4000rpm runs shows no black plugs anymore.  

    one thing this taught me.  When I do my daily I always check throttle travel and choke engagement, but realised I never checked the chokes had returned to minimum. It’s now on the list. 

    Problem solved and lesson learned. Win win. 

    Thank you said by: Bill Hertzel, RotaxOwner Admin

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