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Fuel spitting out of air filters at full power

in 912 / 914 Technical Questions 12 years ago

  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    Gentlemen,

    Since posting the question, i found a rotax guy brave enough to stand in front of my pusher engine when running hard enough to replicate the problem. He confirmed with the air filters removed that he saw a plume of fuel droplets coming out of the carbs when running above 4500. His conclusion was that the since everything else seemed to be in order, the carbs were set too rich and that we should set the needle one notch leaner. At the factory setting, the plugs were quite clean, but the exhaust was sooty.

    I flew the aircraft on the new setting this morning and can confirm that the smell was not there and there was no visible trace of fuel around the air filters after the flight. The engine ran fine, but the idle was a little low and will need bringing up. We will watch the EGT closely and will pull the plugs often to check the mixture, but first impressions show good power and it may have solved the problem.

    For further consideration, the factory prop allows my engine to run about 5380 on climbout and although i like the idea of overlapping valves, this may not b the case on mine. For me the mixture didn't jump to mind because i would have thought that the needle would be shaped to allow good burn all the way through the rev range. Maybe this isn't quite the case.

    I fly most days so it wont be long before we'll know if this mixture adjustment has resolved the issue, so I'll revert with findings soon.

    Thanks for your comments and ideas.

    Neil
      
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  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    Hi Neil,

    I doubt the factory needle position was the real issue and you may have only put a band-aid on the real problem. To change the needle position you would need to calculate your field elevation, normal cruise altitude, prop pitch and have EGT readings on the cylinders. The 5380 on climb out for a ground adjustable prop is a little flat and will give you good climb and it may be even a little too flat and you may be loosing some climb performance. You will surly loose cruise and fuel. If it is a fixed pitch prop you may want to consider another prop. We try to find a balance with our prop pitch and engine rpm that helps balance all our flight characteristics. If you have an in flight adjustable prop then you can make changes at will for the best performance during different flight stages.
    I would suspect a carb sync and or float level issue for my first checks. How old is this engine in years and hours? When making changes to the carbs a sync is usually in order. The exhaust may be sooty if you have been idling a lot and same with the plugs so this may not be a good indicator by itself without other things being factored in.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    Hi Roger,

    The field elevation is 1,000ft ASL and normal cruise is 2500ASL. The prop (as well as engine) has not been touched since the machine left the factory 9 months ago. We now have 110hrs on the engine. I have an Arplast prop which i guess has ground adjustment, but no idea how to do it. I should also point out that this engine/prop is attached to a gyro which is a high drag system and with a Vne of 100 knotts, so maybe i'm not losing any practical cruise performance.

    Whatever the problem, it has been this way since the first flight hour and both carbs/filters do exactly the same thing. The carbs have been mechanically synced (or at least checked) several times. The both open and stop at exactly the same time. I am open to further ideas and advice of course, but a little disappointed that a new engine would leave the factory set up this way.

    Thanks,

    Neil
      
    Report

  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    Hi Neil,

    If these carbs have not been pneumatically synced that could easily be your problem.

    If you are that low an elevation and average flight altitude then you probably should have left the needle clips in their #3 position. The mechanical sync doesn't mean a thing unless you are setting these carbs up after some maint. that would cause them to have lost the original set. A mechanical sync only gets you in the ballpark and even that is down in the idle range. It's just a rough adjustment until you can pneumatically sync them. Those carbs absolutely need to be pneumatically synced and should be every 100 hrs. If you adjust the idle they need gauges on them to make sure they stay in sync because turning one idle stop screw will throw them out of sync at idle. I never adjust any carbs without a set of gauges on them. The engine doesn't care what you have it mounted to. If you give me a call I can tell you how to adjust your prop. It usually takes me 30-40 min. to re-adjust a prop start to finish depending on what it's on. It is easy and I can tell you how and if you don't like it you can put it back where it was. I flew Gyro's for 3 years so I'm familiar with them and their systems.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    Hi Roger,

    Thanks again for your thoughts. I don't really understand how unsynced carbs can cause this problem and show exactly the same on both carbs, but i will heed your sage words and make sure we get a vac guage on them asap.

    I have just looked at the POH for the machine and it says revs should be at 5200 - 5400 at 100% power during a 60 knot climb. I have always been in that range (hot or cold, light or heavy), so should I still consider changing the prop?

    Thanks,

    Neil
      
    Report
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