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Hi guys

I wonder if I could pick your brains regarding a tech problem with my free air 503 DCDI?

I rebuilt the engine last summer including a complete lower end strip down and inspection. I decided the crank still had life in it but replaced the pistons, gudgeon pins, roller bearings and rings, together with all seals and most rubber parts. I also fitted a new stator as the old one had gone high resistance on one ignition circuit. The carbs were rebuilt with new jets, needles and seals and the Mikuni fuel pump was also treated to a new set of gaskets and diaphragms. The crankcase was given a pressure test before the engine was refitted to the airframe.

The engine was put back into service using the Rotax break in procedure, initial flights were fine and I was pleased to see an extra 400 rpm gained. The aircraft was test flown and a new Permit issued. Starting was eager at first, but then needed several strokes of the recoil handle to get going. The heads were retorqued at 2 hours as per the manual.

With about 3 hours on the engine I noticed a momentary hesitation on full power climb out. I thought it might be ice. At 4 hours the same occurred but more noticeably. Then at 5 hours when climbing out from a 'touch and go' the engine lost almost all power at 300 feet and I had to EFATO into a field. The engine continued to run and gave me a bit of help setting up the approach but there was insufficient power for climb and I judged that power was probably insufficient for level flight too.

Fortunately no damage was done (see pic attached). Thinking I had an air leak somewhere I re-torqued everything again and checked the carb rubbers. Everything seemed ok. I don't think the air filters were over oiled. The fuel filter was clean. There was no obvious evidence of carb ice. (It was a hot summers day and conditions like that have given me ice in the past - in addition the electric carb heat may not have been fully functional). I tightened all the fuel system connections. Ground runs to full throttle seemed fine, although some fuel spilt from the carb vent pipes which I put down to the rough field.

I took off successfully from the field and made a short circuit back to the runway.

Back at base I took the heads off to check for evidence of any seizure on the cylinder barrels but all was fine. However on a subsequent test flight, climbing at full throttle, the problem repeated itself. It sounded like the engine was bogging down and revs dropped from 6300 to around 3000 rpm. Looking at the EGT I could see that only one cylinder was affected and was indicating about one third normal temperature. The other was slightly above normal. When the throttle was reduced to cruise descent, normal running resumed.

Since then I have done numerous ground runs and more air tests at full throttle but have not been able to replicate the problem. The removable Rotax fuel pressure gauge tells me that there is a slight overpressure at full revs with both the Mikuni and my electric Pierburg pump running (which is my normal practice on landing and take off). Rotax advise that pressure should be 0.2 - 0.5 bar but I'm getting 0.6 - 0.7 at wide open throttle. With this in mind I'm inclined to fit a fuel pressure regulator or plumb the pumps in parallel rather than in series. I have re-cleaned the carbs and fitted new carb floats in case the old ones were being overwhelmed.

Do you think the symptoms described could be caused by excess fuel pressure?

I've not been able to find anything else wrong or out of spec. I suppose it could just have been something as simple as a piece of dirt playing around in one of the carb jets. Do you think there's anything I haven't considered?

Best regards and thanks in advance,

Tom Brearley
GMTNU-EFATO.jpg (You do not have access to download this file.)
  • Re: 503 power loss

    by » 8 years ago


    Tom by any chance do you have one of those hand primer bulbs???

  • Re: 503 power loss

    by » 8 years ago


    Tom,

    Sounds like a trash problem to me. I have run several aircraft with different types of electric pumps plumbed in series and have never had a fuel pressure problem. You might check the main jets and the float jets for trash. If either of these is partially blocked, the engine will run great at low speeds but not at higher RPMs. overpressure could cause a rich mixture but those pressures you indicate with both pumps running shouldn't be a problem. Overpressure would also cause the carb bowl vents overflow.

    I have seen cases where removing a carb for cleaning would dislodge rubber or trash in the fuel line connecting to the carb. Sometimes with trash, the problem comes and goes as you describe.

    Bill.

  • Re: 503 power loss

    by » 8 years ago


    Thanks Bill. I am trying to decide whether the over-pressure fuel is a red herring or not so your comments are helpful.

    Nick - I do indeed have a primer bulb but it's plumbed in parallel with the electric pump so if it were to restrict flow the fuel should still be able to get through the pump without problem. Do those bulbs tend to create debris too?

  • Re: 503 power loss

    by » 8 years ago


    Tom,

    One thing to look for in the primer: just about all fuel lines are alcohol resistant these days but a lot of the primer bulbs are not and can disintegrate into rubber bits inside if you use alcohol laced fuel. Bad news for fuel filters and float valves. If your fuel filter is downstream of the bulb however, it should catch the trash.

    Bill.

  • Re: 503 power loss

    by » 8 years ago


    Seeing yours primer is plumbed in parallel as you say Tom it should not hinder flow apart from as Bill says...bits of debris,

    I know quite a few guys with older 2 strokes that have had engine outs due to these primer bulbs with the outlet valve sticking part open part closed...indeed it happened to me on climb out at around 300ft....I was lucky to get away with it...on full throttle it would dies...ease back on the throttle it would run ok,

    The only clue I had was that the bulb felt odd when pumping it....that should have prompted me to look further but I didn't,

    Took mine off and prime it with an other method.

    Nick.

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