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  • Starting an IO540

    Since new I've had no issues starting my IO540. Usually, I prime for 5 seconds, leave the mixture full rich, and it fires on the third blade. Until now.

    Yesterday, it took about 8 seconds of cranking before it reluctantly fired a couple of times, spluttered, and finely settled into a finely tuned hum.

    The temperature was -2°c (that's a New Zealand summer for you ). But I also noticed that the density altitude was -2000ft. I got to thinking, is this a temperature thing, or a density altitude thing ?

    This morning the temperature was also -2°c and the density altitude was -1800ft. I decided to prime it for 10 seconds, and did a flooded started with mixture at idle cutoff. If fired on the third blade and burst into life.

    Incidentally, I'm convinced that the idle mixture is set too lean for ISA conditions (there's no RPM rise when I lean it with a high QNH).

    How does everyone else start their engines when it's cold ? Should I be priming more for lower density altitude?
    Last edited by Nev; 06-23-2022, 02:44 PM.
    Nev Bailey
    Christchurch, NZ
    Builders-log
    YouTube Bearhawk Blog

  • #2
    You'll always need more fuel for a cold start. 10 seconds isn't even flooded yet in the cold. We used to tell new guys to prime the pa31s by priming until they felt there'd be fuel coming out the cowling.

    Another option is electronic ignition. Lights much more off mixtures and poorly atomized ones.

    Comment


    • #3
      I remember starting my 1973 chevy impala when it was -20C (-5F). I'd set the choke and pump the gas pedal 10-12 times in order to get it to fire. Summer was different. When it's warm it wasvdifferent. We did those things to get a combustable mixture inside the cylinder.

      My view now is a drop of Fuel will produce some vapor during that time between the prime and the push of your start button. Cold dense air in the cylinder has more O2 atoms which leans things out too. So it normal to prime more to start during cold temperatures to obtain that combustible mixture.
      Last edited by Bcone1381; 06-24-2022, 07:42 AM. Reason: corrected temp to read -20C not 20C
      Brooks Cone
      Southeast Michigan
      Patrol #303, Kit build

      Comment


      • #4
        Cold weather or density? Yes!
        The colder and thus more dense air will require more fuel to create the same mixture.
        For example, our Citabria likes two pumps of prime in the summer but four in the winter and sometimes more in the bitter cold winter.
        Starting year 3 of my 2 year project!

        Comment


        • #5
          So is the air density (density altitude) the main factor, and the cold air is increasing the density (lowering the density altitude)?
          Nev Bailey
          Christchurch, NZ
          Builders-log
          YouTube Bearhawk Blog

          Comment


          • AKKen07
            AKKen07 commented
            Editing a comment
            Perzactly! That’s assuming your airplane/engine doesn’t have any unique contributing factors.

          • Nev
            Nev commented
            Editing a comment
            Great. Makes sense now why a flooded start works in many different temps/density's.

        • #6
          Originally posted by Nev View Post
          How does everyone else start their engines when it's cold ? Should I be priming more for lower density altitude?
          I think this question / discussion depends a lot on the fuel application (injection vs carb), fuel line plumbing, priming technique (primer vs pumped fuel), fuel pressure and flow rate delivered by said system, weather on the day, etc etc.

          Fortunately, there is a large collection of IO-540s with similar fuel system plumbing, similar fuel servos, and similar pumps - all within NZ. So we have a rather good comparison group here.

          Cold start (see my POH available on this forum):
          set mixture at idle cutoff
          pressurize the fuel system by running the pump until fuel flow shows 0 L/hr
          set the throttle for idle
          push the mixture forward to prime for 5 to 6 seconds
          pump off (leave the mixture full rich)
          engage starter

          Never failed - probably over 1000 starts (unless the battery is flat or the engine is hot). I have recently found 4 seconds is enough prime in summer weather, but wintery weather needs the full 6 seconds.

          Comment


          • #7
            Originally posted by Battson View Post

            I think this question / discussion depends a lot on the fuel application (injection vs carb), fuel line plumbing, priming technique (primer vs pumped fuel), fuel pressure and flow rate delivered by said system, weather on the day, etc etc.

            Fortunately, there is a large collection of IO-540s with similar fuel system plumbing, similar fuel servos, and similar pumps - all within NZ. So we have a rather good comparison group here.

            Cold start (see my POH available on this forum):
            set mixture at idle cutoff
            pressurize the fuel system by running the pump until fuel flow shows 0 L/hr
            set the throttle for idle
            push the mixture forward to prime for 5 to 6 seconds
            pump off (leave the mixture full rich)
            engage starter

            Never failed - probably over 1000 starts (unless the battery is flat or the engine is hot). I have recently found 4 seconds is enough prime in summer weather, but wintery weather needs the full 6 seconds.
            I don't think I've ever seen a bendix FI started like that. POH on just about everything I've seen (certified) is the same up to the leave mixture rich, pump off. Usually it's leave the pump on, mixture idle cutoff, throttle cracked, starter until it catches and then bring the mixture in a bit to get fuel flowing, but not flood it.(1/3 to 1/2 in is usually about where it's at) Full mixture would flood most things.

            Comment


            • Battson
              Battson commented
              Editing a comment
              That sounds more complicated and a lot more fiddly.
              Like I said, it works every time
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