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rich mixture, plugs black

in 912 / 914 Technical Questions 8 years ago
914 UL using MOGAS.

Plugs NGK DCPR 7E with .65mm gap.

During the 50 hour check, plugs removed were sooty black.

Engine run throughout according to ROTAX manual and mag drops all within limits.

Was wondering if the carburetors' mixture idle screws should be turned in 1/4 turn before balancing ? Any thoughts please.

HR

  • Re: rich mixture, plugs black

    by » 8 years ago

    Dry black sooty plugs on some cylinders can be normal and is usually due to idling. The lower the idle the worse bit can be. This goes away at higher cruise rpms. This said you do need the have the idle mixture screw set correctly and carbs synced.

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

      
    Report

  • Re: rich mixture, plugs black

    by » 8 years ago

    Thank you Lee for your swift reply.

    I followed Rotax-Owners advice regarding "just" ridge oiling the air filter paper; so as to minimize airflow resistance to the carburetor.

    Returning to the idle mixture setting, I was wondering if there was anything beyond the undoing the mixture screw 1 & 1/2 turns from the stop position. Is there any in house educational video on refining this for the Rotax with it's 2000 rpm tick over ? In my past life, I used to view the colour of the flame via a see through spark plug which had an angled mirror at the other end and I would then adjust the mixture screw accordingly to get the right colour at the sweet spot (remember ?) - however this was normally done at an idle of approx; 800rpm. I can see a vibrational challenge here doing this on the Rotax 912UL. Hence, my post for help.

    In the absence of any further suggestions, I will be tempted to turn the idle mixture screw in by a 1/4 turn to weaken the mix. Obviously I would be watching out for any rise in any cylinder temperature.

    Thank you for your kind attention.

    HR
      
    Report

  • Re: rich mixture, plugs black

    by » 8 years ago

    It isn't just the mixture screw. It is in part due to the unequal length of the air intake to eac cyl. This really isn't anything to be concerned about. It's pretty normal.

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

      
    Report
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