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912ULS EGT and needle clip position

in 912 / 914 Technical Questions 11 years ago
I have a 912ULS with about 270 hours.

I fitted an EGT guage (probes in the 2 front pipes), and found that the EGT is low at lower cruise rpm. The EGT looks good at full power. I am wondering if it is worth changing the needle position - I understand this mainly affects the mixture at lower power, not full power.

My EGTs are:

Full throttle at around 4000' density altitude:
(RPM L/R degF L/R degC)

5500 1400/1350 760/730

Part throttle at about 9000' density altitude (it's summer here):

5200 1350/1275 760/730
5000 1250/1200 680/650
4800 1200/1150 650/620
4600 1200/1150 650/620

Is it worth trying a different needle clip position? Or should I be looking for something else e.g. worn needle/needle jets? Or are these in the "OK, just leave it alone" range?

What causes variations in the mixture from one installation to another? My first inclination is that Rotax know what they are doing and I shouldn't change anything, but it would be nice to reduce fuel consumption and get a bit more range at times.

Thanks,

Andrew Rowley

  • Re: 912ULS EGT and needle clip position

    by » 11 years ago

    Hi Andrew,

    A couple of things can affect the EGT temp reading. First is the distance from the outlet port on the exhaust. Too close makes them hot and too far makes them colder. 100MM or 3.9" is good. Carb sync, fuel air mixture, air leaks, ect.... Your EGT's look normal. Each plane and engine is slightly different. A 50F spread like yours isn't any big deal and is perfectly normal. You can see anywhere from the same reading on both to 80F for a normal spread and rpm will most definitely affect the temp spread. If you see more than 115F constantly then something should be looked at. If EGT's are within 20F consider them the same (more or Less). The higher in altitude you go the lower the EGT temps.
    I wouldn't change anything at this point.

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

      
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  • Re: 912ULS EGT and needle clip position

    by » 11 years ago

    Thanks Roger.

    I'm not concerned about the left/right spread, they are consistent enough that I think they are more likely to be variations in the guage/probes than a real difference.

    I'm more interested in the difference to what the manual says is normal. The manual says normal operation is 800C/1470F. Full power is close to this, but 5000rpm cruise well below. I sort of expected that full power would be richer therefor lower EGT than cruise power - although the manual does suggest the opposite i.e. it lists a higher EGT at takeoff.

    My probes are probably closer than ideal, I was working from an old copy of the manual which said 70mm from the exhaust flange so that is where they are. I would expect a hotter reading rather than cooler.

    The main concern I suppose is whether I might end up with problems due to running too rich e.g. deposits building up. However, if the EGTs are normal I am happy to do nothing. The manual isn't too specific about the change with different RPMs - although come to think of it, the normal EGT is 80C below max at takeoff, which is what I see - but with lower temperatures.

    Regards

    Andrew Rowley
      
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  • Re: 912ULS EGT and needle clip position

    by » 11 years ago

    I find that it comes off the needle and onto the jet at 5000. Then the exhaust temps drop off as it becomes richer at this rpm.
    My rear temps are 50 c hotter than the front so be careful here going by the front only.
    I think "cooling" airflow over the front exhaust pipes may distort readings sometimes.
    A low speed, high power climb should max out your temps.
      
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