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All plugs black velvet/Excessive RPM drop both ignitions

in 912 / 914 Technical Questions 7 years ago
2015 RV12 SLSA with 912ULS, 130 hrs. History: Went to start, fuel blowing out of overflow on #2 carb. My A&P somewhat familiar with Rotax, advised float issue, since aircraft annual due in month, carbs require 200 hr overhaul, both decided to just send both carbs to Lockwood Aviation for overhaul. Reinstalled, initially ran ok, challenges getting carb synch completed. Engine run-ups deteriorated after intial test run up. Started to get over 300 rpm drop on each side on run up at 130-140 degree engine temp, rpm drop reduced to about 200 rpm per side at engine temp 190F. Ran ok in flight, produced respectable power, but synch just not perfect. Removed plugs, all black velvet color, no wet appearance. Reinstalled new plugs from Lockwood, .028” gap. (A&P indicated initial reinstall of carbs choke setting not right, could have caused rough engine and spark plug issue). Started perfect, did runup at 130F, approx 70rpm drop each side. Taxi for takeoff, additional run up, all good. In flight, noticed carb synch ok, not perfect. All engine parameters within limitations, normal. Landed. Did run up, rough engine, RPM drop near limitation. Observer indicated during my initial runup, light exhaust smoke noted. At this point, not sure where to go. All spark plugs removed during annual inspection, prior to rebuilt carbs, perfect condition.
Any suggestions?

  • Re: All plugs black velvet/Excessive RPM drop both ignitions

    by » 7 years ago

    Couple of things here and do simple most common causes first.

    Having a couple of black DRY sooty plugs is very normal after you have been idling. If you want to check plug color go fly and when you land turn it off and don't idle for a long time or just run it up on the ground to about 4K for 4 -5 minutes and immediately shut it down. No long idling.

    Before you do this the plug gap should be between .023 - .027. It's in the Line Maint. manual. The wider plug gaps can cause your soot. I usually just use .025 since it's in the middle. Personal preference. Make sure you apply your thermal conductive paste to the threads and don't get it down on or even near the electrode.

    Carbs don't need to be overhauled at 200 hrs. It is just an inspection. There is nothing in print when they are required to be fully overhauled. Just to rule out heavy floats I would weigh them which should now be done each annual. No more than 7 grams per set. The carb sync should be fairly easy to set dead on. You may have an air leak on the air intake carb rubber flange or even where the air intake screws onto the top of the head. Just double check it. While you're at it you can take a couple of minutes and double check the mixture screw on the neck of the carb for 1.5 turns out.
    If it ran okay and started to act odd again double check your carb sync since you said it was a challenge.
    Since someone said they saw some smoke go a head and do a differential compression test. Just rule it out. Don't leave stones un-turned.

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

      
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  • Re: All plugs black velvet/Excessive RPM drop both ignitions

    by » 7 years ago

    Thanks Roger.
    Both Carburetors were rebuilt by Lockwood Aviation, Sebring,FL..all set up like factory new. My aircraft had annual by A&P, all compressions perfect. It wasn't a few sooty plugs, but all 8 sooty and looked identical. Note: during annual, prior to running engine with rebuilt carbs, all existing spark plugs were perfect colors, but we installed new plugs anyways. .028" gap, since this is what Lockwood Aviation sets them at when they send out to customers. Also note, the fuel running through my engine is unleaded, 93 octane, no ethanol. Could my mechanic have installed/adjusted choke connections incorrectly? How tell? So, could this spark plug gap and sub par carb synch cause this rough running ignition check and sooty plugs? Thanks again. I am very challenged here in North Georgia finding any qualified/experienced Rotax technicians and on top of that my RV12 is SLSA.
      
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