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  1. #1
    Co-Pilot goflygaz's Avatar
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    912 Fuel Pump Replacement - Fuel Pressure

    I have been asked many times recently about the fuel pressure testing I did a month or two ago. So having decided previously not to post this, It seems that several people have an interest in this.

    Please note: the attached is my findings and represents my opinion only. I have provided this info so you can decide for yourself.

    Conrad at Skydrive has been very helpful, but Rotax say there is no issue, please no bashing of the UK suppliers. If you have a problem, contact Skydrive and let them know with full details.
    The BMAA have now approved the minor mod and I have had no issues since it was fitted.

    Fuel Pressure Investigation on Rotax 912ULS 28th Feb 2014b.pdf
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  2. #2
    Captain jjones's Avatar
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    very interesting Gary.

    what are the implications of installing the regulator other than the minor mod, is there any maintenance for the regulator and what is its replacement period?
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  3. #3
    Co-Pilot goflygaz's Avatar
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    No maintenance - just set the pressure initially.
    I have never seen one with a damaged diaphragm, but I intend to replace after 4 years.
    The linked document shows the mounting and considerations when I applied for the Mod.
    Attached Files Attached Files
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    Trainee Pilot Dave Allen's Avatar
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    I recently changed a pump from the Pierburg to the Corona, but on an older 912. This is fed directly to the carbs, with no "bleed off" to the tank, so is seeing full pump pressure, including the fluctuations. I have not noticed any significant additional fuel burn (no flow gauge fitted), but the pressure is interesting, since the fitting of this pump, the 912 starts very easily from cold with no choke at all.....
    Also, how much engine oil would you expect from the new "drain" hose - I'm thinking of fastening a bottle under this to monitor it, as there is always a dribble after flight.


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    Co-Pilot Sean Dougan's Avatar
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    Probably coz the weather is warmer? I note the C42s at the field all start easier but they haven't had their pumps changed.

    The float bowl provides a disconnect between the pressure of the fuel fed to the carb and what actually goes into the engine.
    I understand the valve has been tested to be good for about 0.8bar. The pump is giving a little over half that.


  6. #6
    Co-Pilot goflygaz's Avatar
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    Quote Originally Posted by Sean Dougan View Post
    Probably coz the weather is warmer? I note the C42s at the field all start easier but they haven't had their pumps changed.

    The float bowl provides a disconnect between the pressure of the fuel fed to the carb and what actually goes into the engine.
    I understand the valve has been tested to be good for about 0.8bar. The pump is giving a little over half that.
    I doubt the starting is affected by the change in pump, almost certainly a 'placebo' effect.

    Whilst the valve may have been tested at 0.8bar - was that a new or worn valve, was the carb subject to vibration in the same way as a running engine?, also the pressure it first 'cracks off' against may be different to what it is able to close against.
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    Captain Frank Thorne's Avatar
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    Interesting reading... Pity you havent got data for the old pumps as a comparison. I do know mine creeps up to around 0.4 bar on start up and then drops to 0.3 in flight and never moves unless at max revs then it drops a bit lower. Correct me if I am wrong but I thought 0.3 bar was optimum with min at 0.2 and max 0.4. If you regulated yours at 0.17 to 0.27 isn't that a little low?

    If you fitted a regulator doesn't that negate the use of Fuel pressure guage? If you have a regulator in line and theres a filter problem then you possibly wouldn't get any indication until the engine stopped. Overpressure wouldnt indicate at all............


  8. #8
    Co-Pilot goflygaz's Avatar
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    Quote Originally Posted by Frank Thorne View Post
    Interesting reading... Pity you havent got data for the old pumps as a comparison. I do know mine creeps up to around 0.4 bar on start up and then drops to 0.3 in flight and never moves unless at max revs then it drops a bit lower. Correct me if I am wrong but I thought 0.3 bar was optimum with min at 0.2 and max 0.4. If you regulated yours at 0.17 to 0.27 isn't that a little low?

    If you fitted a regulator doesn't that negate the use of Fuel pressure guage? If you have a regulator in line and theres a filter problem then you possibly wouldn't get any indication until the engine stopped. Overpressure wouldnt indicate at all............
    Fitting a fuel pressure gauge is still probably a good idea, as you say, early warning of a problem perhaps.
    When you say old pumps for comparison, I did test the old AC pump , I guess you mean the previous one - sure none to hand at the time, but suspect they work similar to the AC. We have to remember that these are automotive carburettor fuel pumps - and cars for a very long time no longer have carbs fitted and are all high pressure EFI. So I guess Rotax have found their choice of suppliers dwindling for such items.

    I have chosen to regulate mine to 3.5psi (0.24bar) nominal pressure, which does give slightly lower min peak values - but am happy that with a 47 litre/hr (at full revs) spare flow capacity all should be well.
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  9. #9
    Co-Pilot Sean Dougan's Avatar
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    Sorry I'm late getting back to this, Gaz. Pulling long days and haven't been in the right mind for other stuff.

    Regrettably I couldn't say what was involved in the testing of the carby. The information I got was second-hand. I think, though could be wrong, that it was Conrad who carried this out.

    Something I've thought about is that there can be a rise in the fuel level in the bowl with an increase in supply pressure; the floats would become more submerged before their buoyancy provided enough force to shut the valve.
    I figure a way to check that would be run the engine up to the high fuel pressure point and kill it. Then carefully pull a bowl off the carb, see where the level sits and compare with the specs. I wonder if winding up and down that regulator you've fitted would give any noticeable difference?

    The floats can get porous and sink, the lever arms wear and so can the valves. It could be a lot of the problems are merely caused by carbys that are forked, but have been adequate until the pump change.

    There's a E-Fox here that at first from new suffered badly from float bowl flooding. It had everything going wrong for it - tail-dragger so the carbs sit at a wild angle on the ground, along with the reversed manifolds so they get full brunt of any vibrations, and this new pump.
    It was fixed by changing the springs inside the float valves, which provide a buffer between the tail attached to the float lever arm and the valve head. The bit I still haven't quite figured out was the springs fitted were weaker than the originals.


  10. #10
    Co-Pilot goflygaz's Avatar
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    Quote Originally Posted by Sean Dougan View Post
    Something I've thought about is that there can be a rise in the fuel level in the bowl with an increase in supply pressure; the floats would become more submerged before their buoyancy provided enough force to shut the valve.
    I figure a way to check that would be run the engine up to the high fuel pressure point and kill it. Then carefully pull a bowl off the carb, see where the level sits and compare with the specs. I wonder if winding up and down that regulator you've fitted would give any noticeable difference?

    It was fixed by changing the springs inside the float valves, which provide a buffer between the tail attached to the float lever arm and the valve head. The bit I still haven't quite figured out was the springs fitted were weaker than the originals.
    You are quite right that increased fuel pressure has a marginal effect on float bowl fuel height and therefore will richen and lean slightly - the difference based on past experience on different carbs is that it is negligible. The best way to test is by measuring the air/fuel ratio in the exhaust whilst raising and lowering fuel pressure. You method will work in theory, but in reality I doubt you can remove the float bowl and measure the remaining fuel level with any degree of accuracy.

    Your last point is interesting as I specifically asked about stronger springs and was told nothing like that exists.
    If you are coming to Cromer today we can discuss more over a beer or two.
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