Hello all,

I recently purchased a used Phantom Ultralight with a Rotax 503 DCDI with dual Bing 54's. I decided to go over it and replace/rebuild several items before getting it airborne. Before doing any work on it, I noticed that at idle sometimes, it would slowly die after idling for 30-45 seconds.

So, I replaced all the fuel lines (including the primer lines and the bowl vent lines, fuel filter, cleaned out the metal fuel tank, rebuilt both Bing 54 carbs, rebuilt the Mikuni fuel pump and replaced the propeller (another WarpDrive 2 blade).

I did a basic carb synchronization (set both pistons to bottom stop, set idle screws 2.5 turns in from initial contact with carb piston, and checked for equal piston movement with throttle increases).

I set the new prop at 8.5 degrees to start based on input from the WarpDrive factory.

Went for first start, took me a little bit of tinkering to get it started, but once started, it ran well. The only problem was that the prop pitch was so insufficient that it would not generate enough thrust to even move the ultralight, and would easily hit 7000 rpm if I had of pushed it that hard.

I loosened the prop up and this time set it for 11 degrees of pitch, thinking that the needed setting is likely going to be somewhere between the 8.5 degrees and the 11 degrees. Started it back up, but now it will only idle (around 2000 rpm at best). If you move the throttle forward at all, it starts to bog down and die.

To me it seems like the main jet needles may need to be set a little richer (although they were set to spec when I rebuilt the carbs). For the record, I have a 158 main jet, 45 idler jet, 2.70 needle jet, 11K2 jet needle with the clips set at position 2. I am located in the Mineola, Texas area so I am around 500' MSL (although today the density altitude is 2,740' +).

Anyone have thoughts on what might be causing it to bog down when throttling up even though it ran fine before I changed the pitch on the prop?

  • Re: Rotax 503 Acceleration Problems

    by » 5 years ago

    Remove the exhaust manifold and inspect the pistons, cylinders and rings. Stuck rings, scuffed piston or crankcase leaks can cause the issues you have. 

  • Re: Rotax 503 Acceleration Problems

    by » 4 years ago

    Joel and Dennis,

    The issues above also apply to my 503.  The engine and dual Bing carb setup and parts and pieces Joel described  match my 503 as well.  The only difference is i have  a 3 bladed ultra-prop set at 12 deg. pitch.  Ultra-prop recommended a 13 deg pitch, but engine was not achieving full RPM at full throttle.

    On the last static run, tied down,  max RPM was 6150, at full throttle.  Is 6150 what should be achieved in a static test?

    Also,  There seems to be a slight surge in RPM just above 4000, where the engine rpm accelerates faster then the throttle is advanced, or there is a delay and the RPM advance is not linear with the throttle setting.  The  RPM at which this happens is approx. at the transition point from mid-range to full.

    Also,  The EGT's do not get up to the 1000 deg point.  best has been closer to 800, on an 80deg, Louisiana day.

    Any comments on the above are welcome.  The plane is a Hi-Max and is yet to get off the ground pending engine  running correctly!



  • Re: Rotax 503 Acceleration Problems

    by » 3 years ago

    John was it solved? ... the same thing happens to me. Greetings Raul


    John se solcuiono?... me pasa lo mismo. Saludos Raul

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