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High RPM Roughness

in 912 / 914 Technical Questions 12 years ago
Yesterday after a normal warm up and departure, while climbing through 1500" the engine suddenly began to run rough with an Rpm drop. I pulled back the power and headed back to where I just left. It smoothed out around 4800 RPM but the roughness recurred when I increased throttle to full power. I landed without incident. On the ground it would pull 5200 RPM which is about my static rpm for about 15-20 seconds and then go rough again. Fuel pressure remained normal and addition of electric fuel pump made no difference. I placed the plane back in the hanger and drove to my destination.
I'll start by examining the easy things; fuel filter, plugs, carb. Any specific idea where I might look to efficiently find my problem?
Thanks

912 ULS 120 hours, Kiev prop

  • Re: High RPM Roughness

    by » 12 years ago

    Hi JD,

    Doesn't sound like a filter problem because the pressure is staying in the normal range. Two things you might check are carb balance and EGTs. Unbalanced carbs could cause rough running but this is not usually a problem that happens suddenly. Imbalance also is usually is more pronounced at low speeds rather than high speeds. It could be trash in a carb. If one carb is running lean at WOT from a partial blockage, it would show up as higher EGTs on that side even though manifold pressure is good. If the engine runs well for 15-20 seconds before getting rough I would strongly suspect trash in the carb float valve or the main jet. Could be just enough of a blockage to allow the engine to run normally until maximum fuel is needed.

    Just guessing, but I hope it helps.

    Bill.
      
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  • Re: High RPM Roughness

    by » 12 years ago

    Hi JD,

    Pull the carb bowls and look for a little debris. Put solid color rag under each carb with the bowls off and flow a small amount of fuel to make sure it is clean. The solid colored rag makes it easy to see any debris. (i.e. white)

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

      
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  • Re: High RPM Roughness

    by » 12 years ago

    Also, take a peek at your carb flange sockets. They could be loose or worn out.
      
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  • Re: High RPM Roughness

    by » 12 years ago

    Ignition? Break down of sparkplug(s), or lead(s)

    Pud
      
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  • Re: High RPM Roughness

    by » 12 years ago

    Hi JD

    What engine is it and how many hours has it flown. Also have you had any maintenance done recently or have you altered anything.

    I would go with Rodger and Bill on this and check you carb bowls first, I have had flakes of plastic off the floats that just hang around in the float chamber and when fuel demand is high they get sucked up and block the main jet.

    Good Luck
      
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