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Fuel Mixture on 912ULS

in 912 / 914 Technical Questions 14 years ago
I just pulled the spark plugs during my first annual inspection on my PiperSport. The plugs for cylinders 2 and 4 look great (gray upper and lower). The plugs for cylinders 1 and 3 are black charcoal upper amd lower which looks like the right carb is running rich. Is there any reason to be concerned? The engine seems to be rinning great. I normally fly at 5250 RPM and fuel burn is 5.5 GPH.

Is there a way to adjust the mixture on the carbs?

  • Re: Fuel Mixture on 912ULS

    by » 14 years ago

    Quickest and easiest things to check before you tear into the carbs would balancing and enrichener (choke). If balance is off, one side can blacken the plugs and idle roughly but will run fine at cruise. If the enrichener is not closing fully on one carb after starting, that carb can run rich. Both are easy fixes. Are your EGTs a little lower on the side with black plugs? This would also indicate a rich carb.

    Bill.
      
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  • Re: Fuel Mixture on 912ULS

    by » 14 years ago

    Yes it is. About 30 degrees cooler.
    Thanks for your comments.
      
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  • Re: Fuel Mixture on 912ULS

    by » 14 years ago

    Hi Richard,

    It is normal to see a dry black soot from time to time on 912 plugs. It may even change to different plugs from time to time. It happens at idle and it isn't happening at the higher rpm setting in cruise. It is a mixture issue with the air intakes and for the most part nothing needs to be done. If you want to prove it to yourself run the engine up to 5000 for a minute or two and shut it off once. Then pull the plugs. Those plugs will not be black. You have different length air intakes to each cylinder which plays a part, you may have two separate air intakes and not factory, you mixture screw may not be set at 1.5 turns and even if it is may need tweaking. Most just leave it alone it isn't an issue.

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

    Thank you said by: bill martin
      
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  • Re: Fuel Mixture on 912ULS

    by » 14 years ago

    Richard,

    I think roger is right. If your engine is idling smoothly, running well at cruise, and the EGTs are that close, the black plugs are really not an issue. I think if you run the idle mix screws in until they lightly seat and back them out exactly 1 1/2 turns, then balance the manifold pressure, you will probably see that 30 degree difference go way down. Mine run about 5 degrees apart at 5000rpm.
      
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  • Re: Fuel Mixture on 912ULS

    by » 14 years ago

    Hi Richard,

    Normal CHT temp differences are anywhere from 0-15F and normal EGT temps difference are any where from 0-80F. Max spread is 115F for EGT. My CHT's are 0-10F difference and my EGT differences are 0-40F.

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

    Thank you said by: YEN NIEN YU
      
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