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Rotax 912 carbs overflowing

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  • Rotax 912 carbs overflowing

    As my engine doesn't want to start at the moment every time I try to get it going it pisses a load of fuel out of the overflow pipe. The float bowls are full. Trying to determine how normal this is (did have the fuel pump exchange recently). I don't remember it pissing fuel out before but then it normally starts... I can't see how it might really be avoided, if the engine is vibrating away turning over then the float valve is hardly likely to do a good job of sealing is it?

  • #2
    Rotax 912 carbs overflowing

    I think you're right Gwyn. Its a mechanical pump so if the engine is turning then it is pumping fuel. Its got to go somewhere.

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    • #3
      Rotax 912 carbs overflowing

      yeeeees, it does, but......doesnt Gwyn have a point about the float valve? I can easily imagine the float vibrating around and the shutoff valve dancing, letting fuel through?

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      • #4
        Rotax 912 carbs overflowing

        Are you replying to yourself Laurie or did someone remove an intervening post?!

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        • #5
          Rotax 912 carbs overflowing

          Newer pump has a higher output pressure and can lift the needle valves. I believe there are stronger sprung ones?
          Most three-axis installations have a bleed back to the tank, but I don't think P&M flexwings do.
          Dave

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          • #6
            Rotax 912 carbs overflowing

            Hi Dave,
            You are right, the early 912 P+M trikes have no bleed back but the later ones and all their 912S engined ones do - but even with bleed back my 'new' style fuel pump runs at 0.5bar (old max used to be 0.4bar). No problems to date but maybe I need to enlarge the hole in the bleed back restrictor??
            AndyB

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            • #7
              Rotax 912 carbs overflowing

              Andy,

              I believe 0.5bar is just within the Rotax(Bing) spec for fuel pressure, so best left alone.
              But it does make you see that without the return, pressures MUST be rather too high.

              A mechanical fuel pump also delivers a very 'peaky' flow, and a pressure meter indication probably evens this out, so you may not be seeing the true maximum pressure, that might just be enough to squirt a little extra fuel through.

              The LAA have looked at this after an engine failure during climbout that came back to overpressure and flooding.
              This was on a 'return' systemed 912 installation that had just had the new pump installed.
              I think Jan 14 Safety Spot covered it, but can't find the mag now...
              A call to Malcolm McBride at the LAA might be useful.

              Dave

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              • #8
                Rotax 912 carbs overflowing

                FWIW. A bleed back to tank orifice should not really be able to red'uce the pumped pressure, because its regulator would simply increase the pump's output to sustain the perhaps half bar. What it will do is stop pressure build up when engine off from heat etc. and thus static carb. overflow.

                It's disturbing to find more than one recommendation to drilling out the jet, or generally ignorantly fiddling with the prescribed arrangement. This is not the answer, unless you wish to be an amateur designer.

                Mike hallam.

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                • #9
                  Rotax 912 carbs overflowing

                  Mike, that's assuming the pump has the capacity to provide the extra pressure with an increased return flow orifice. Effectively we're looking at the strength of the return spring behind the pump diaphragm, verses the return flow through the orifice. The new pump must have a stronger spring, so it needs either a bigger orifice to restore the status quo, or stronger springs in the carb needle valves. Mr McBride might have the answer.

                  Dave

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                  • #10
                    Rotax 912 carbs overflowing

                    In theory I supose you might have a point Dave,

                    However if the by-pass orifice is so large as to drop the pump pressure by allowing a more significant flow, your 'regulation' will give varying pressures at different carb. fuel demand.

                    Thus if, say, at mid throttle you have opened the orifice big enough to obtain the pressure you'd like to see, then at WOT there's a distinct possibility of starving the carb's due to its competing petrol discharge route.

                    Vice-versa, should you only open the orifice enough to obtain the desired pressure at full throttle, ergo at low demands the pump can provide sufficient flow to achieve it's "as made" pressure.

                    If in fact the newer pumps do have the reported higher pressure which apparently can overcome the Bing float chambers' cut-off needle valve, there are. I understand, uprated float needle versions available.

                    All I'm saying is picking on one 'obvious' solution to a perceived problem, without considering the fuel flow dynamics is hardly a way to be a development engineer.

                    mike hallam.
                    P.S. I should have added that a normal pump has capacity greatly in excess of the WOT fuel demand, making it even more of a gamble if one simply increases the return line capacity. Perhaps a solution to your needs has already been invented by Rotax & the pump designer ?

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                    • #11
                      Rotax 912 carbs overflowing

                      Hi Mike and Dave,
                      Reading this thread with interest.

                      To recap:
                      My Quik trike operated at a max of 0.4 bar with the original (AC) fuel pump fitted. As it has a 912 engine a return bleed off line is optional, and I removed it (as I was thinking of fitting an auxiliary fuel tank system feeding the main tank). Max pressure remained 0.4 bar. This was the quoted rotax max pressure for my engine so in that configuration all was good.

                      I then fitted the new rotax fuel pump and fuel pressure increased - normally 0.5 bar, occasionally a little higher. Refitted the bleed off system - no difference to pressure - so max is 0.5 bar or just a little over - which is above the original rotax max fuel pressure for my engine.

                      I have made no changes to my carbs and have seen no advice to do so, and to date all seems OK. The engine has 320hrs.

                      What is the current rotax max fuel pressure for the 912 series engines?
                      Is that pressure applicable to all 912 series engines or just recent ones fitted with the new fuel pumps from manufacture?
                      If I should reduce my max fuel pressure to 0.4bar how can I do that (opening the bleed orifice seemed like the only way)

                      AndyB

                      Edited to add:
                      The 'new' pump type is BCP Corona and the Conair web site gives a pressure range for it of 0.15 - 0.5 bar. But no indication if this raised pressure is or is not OK for the carbs on earlier engines.

                      Edited again.
                      Just had a phone call from Nigel Beale to confirm that 0.5bar is OK with all 912 and 912S carbs - so I think that answers everything.

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                      • #12
                        Rotax 912 carbs overflowing

                        Andy's results above would seem to indicate that either pump has plenty of capacity to feed carbs and an optional return bleed and maintain 0.4bar (AC) and 0.5bar (BCP). So Mike's assertion that opening up the orifice to drop the pressure is not a sound way to proceed would seem correct.

                        I will follow this up with the LAA (after the rally) and see what they think.

                        Dave

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