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Rough running at 4200 or more RPM above.

in 912 / 914 Technical Questions 10 years ago
912usl in myRV12
During my last flight I could only get 4700 rpm WOT level flight
I check carburetor bowls only the right one had some debris
After put everything back together and running the engine it run very smooth from idle to 3800 rpm above that it feel like the engine is going to come off the airframe !
I check the carburetor balance and both carbs are even at idle and 3500 rpm above that the gauges drop to near zero so I do not know but again the engine shake too much and it is scary
Help please
Andre

  • Re: Rough running at 4200 or more RPM above.

    by » 10 years ago

    Hi Andre,

    My thoughts would be fuel starvation. Is the fuel pressure good and the fuel flow sufficient at both carbs? It will take a lot to block the main jet but if there was a restriction in the fuel hose to one of the carbs that would cause problems at high power settings.

    Is your fuel pump OK, do you have an electric back-up pump that you could try?

    Have you change anything recently? I always suspect what I did last, even if it appears insignificant.

    If the fault can be replicated on the ground then stop the engine when it is running rough and look at the fuel level in the float chambers. That will tell you which carb is running out of fuel or it could be both of them. I think that the normal fuel level is approximately 6mm below the top edge of the bowl.

    I hope you fix it!

    Kevin
      
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  • Re: Rough running at 4200 or more RPM above.

    by » 10 years ago

    here is a photo of a mainjet with a piece of float in it.

    blockedMainjet.jpg (You do not have access to download this file.)
      
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  • Re: Rough running at 4200 or more RPM above.

    by » 10 years ago

    Thank you will check the suggested items tomorrow and report the findings
    Andre
      
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  • Re: Rough running at 4200 or more RPM above.

    by » 10 years ago

    That's an interesting photo Rob. I take back what I said, it would appear that you can block the main jet ;-)

    I guess that you would have problems on part load as well as full load with a blockage like that.

    Thinking about Andre's problem I wondered whether there might be a fuel vaporisation problem. Is there a fuel return pipe to the tank or are there any hoses running past hot parts or is the engine tightly cowled?

    Kevin
      
    Report

  • Re: Rough running at 4200 or more RPM above.

    by » 10 years ago

    There is a fuel return line to the tank a electric pump runs all the time!
    Will the EGT tell me which carb is the. Problem ?
      
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