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Hello,

I've been dealing with a 2005 912ULS (many different issues over several months time) and the latest is an engine miss below 4,000rpm.

 

Above 4,000rpm it runs very smooth. Throttle response is good from idle on up, but below 4,000rpm you can hear a definite miss and feel it.

The ignition check is good with less than 100rpm drop.

Plugs are new and gapped to .024

The old number 2/4 plugs were dark, indicating being rich.

Carbs have been balanced to within less than 1" pressure difference.

Floats have been weighed and they are good. Float level has been checked and it is good. It actually is acting like a sunk float but they are good. I've dealt with about 20 sunken floats on my engine so I'm versed in what they do.....LOL

Inside of exhaust pipe is sooty indicating a very rich environment.

Enrichment circuit has been checked and is good.

Slide and diaphragms have been checked and they are good.

Air filters are clean.

Has 2 brand new CDI boxes.

Next task is to check spark plug cap resistance and we are ordering new o-rings for the idle mixture screw and idle jets.

 

Any other ideas on what it could be?

Glenn

  • Re: 912UL Miss Below 4,000rpm

    by » 2 years ago


    When was the last time the carbs were cleaned and rebuilt?  If more than 500 hours ago, might want to have that done…


  • Re: 912UL Miss Below 4,000rpm

    by » 2 years ago


    Two things and a couple of things to rule out.

    First I agree with Mike. When were the carbs last overhauled and if they haven't then they need to be with that age. Did you clean out the idle jets? Mixture screw checked at 1.5 turns out? The one next to the main jet needs to be removed and cleaned out. What rpm on the higher side did you use to sync the carbs and are they synced at the higher rpm and idle?

    Then you may want to unscrew the spark plug caps, trim the wire end back about 3/8" to get some clean wire, apply a dab of dielectric grease and screw them back on all the way down.


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


  • Re: 912UL Miss Below 4,000rpm

    by » 2 years ago


    Carbs were cleaned and new rubber installed within the last 100 hours.

     

    Thanks!


  • Re: 912UL Miss Below 4,000rpm

    by » 2 years ago


    First I agree with Mike. When were the carbs last overhauled and if they haven't then they need to be with that age. Did you clean out the idle jets? Mixture screw checked at 1.5 turns out? The one next to the main jet needs to be removed and cleaned out. What rpm on the higher side did you use to sync the carbs and are they synced at the higher rpm and idle?

    Then you may want to unscrew the spark plug caps, trim the wire end back about 3/8" to get some clean wire, apply a dab of dielectric grease and screw them back on all the way down.

     

    I just checked the idle jets and the main jets for debris. They were clean. I went ahead and had him order new o-rings for the idle jets and idle mixture screws. Mixture screws are set at 1.5 turns out.

     

    We are going to check the resistance on the caps, trim the wires back, add the grease and reassemble them.

     

    Carbs were synced at idle, 3,000rpm and they are in balance up through 4,500 rpm where we stopped cause it was getting really windy.....LOL.

    Thanks for the ideas!


  • Re: 912UL Miss Below 4,000rpm

    by » 2 years ago


    The mind can plays tricks on the pilot(s);

    Before I owned it, I used to fly right seat with the previous owner. We had many wonderful flying adventures. There was one period of several months,  where we used to get a miss in our faithful 912 ULS, during cruise climb (5200rpm), usually about 6000ft (probably 3500 above terrain) as we passed over an extended area of "tiger country". 

    We tried new plugs, AvGas shandies, cleaned the carburettors, all to no avail. This phenomena only happened in the one general area, at the one stage of flight - in the end we just ignored it and what do you know? the miss went away.

    We figured our anxiety over taking the risk of a direct line of flight, over very inhospitable terrain, somehow heightened our perception to the point where we separately imagined an engine hesitation/miss. We reinforced this perception by verbalising what we thought we had just felt and then went on to "trouble shoot" without success.

     


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