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Removal and reinstallation of throttle butterfly

in General Tech Discussion 6 years ago

  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    Thanks Roger. Your advice has been most helpful. Both carbs put back together now and the engine run up. It seems however as if the float bowls are still overflowing even though everything has been set per the manual on the bench. What may cause higher than normal fuel pressure? On stripping the carbs, both sides showed equal wear on parts such as floats and the float bracket. On the bench, with the carb upside down, was set to float bracket parallel with the carb body as per the manual. However, when installed on the engine and after running for a short period the float bowls seem to overflow and the float bracket (albeit upside down) feels to contact the fuel valve stop with the bracket arms pointing low towards the ground when manipulated manually with the float bowl removed. In order to get the bracket arms level with the carb body you have to push up against the float bowl spring. Everything has been done in accordance with the manual with upgraded parts etc but still the carb bowl seems to overflow. I have checked on another aircraft of identical type and its float bracket behaves as expected and acheives bracket arms level with carb body with no resistance. I am wondering if all along there has been an issue common to both carbs with too much fuel pressure? What would cause excess fuel pressure (a faulty pump perhaps)?
      
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  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    Did you set the float armatures at 10.5 mm vertical distance from the carb bowl edge or use the Rotax measuring tool?
    If they are truly overflowing then it has to be the needle valve or float armatures not adjusted properly. Heavy floats will cause an overflow to. My money is one or both of the first two listed.

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

      
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  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    I replaced the fuel valves and float brackets in both carbs. The fuel valves were the old red tips so I exchanged them for the current black ones. Both float brackets had bent arms, had been sitting way too low in the float bowls and both had wear on the adjustment tab where the end of the fuel valve had worn a hole seemingly from either excessive vibration or from constantly pushing against the end of the valve. Both carbs were fully stripped on the bench and returned to settings as per the manual and a float bracket level setting tool was used to set the armatures. The floats were weighed and were well within limits. It is just like there is too much pressure in the system. I think it is a pierburg fuel pump. Is it possible for these to go wrong and end up over pressurising the system? In hindsight it seems odd to me that the wear on the old float brackets and the way in which the armature levels had ended up set were identical in both carbs like there was some issue common to both carbs that was causing problems. I have read a few other posts on high fuel pressure and have seen that blocked fuel return lines can cause high pressure? Are there any other possible high pressure causes?
      
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  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    If you have a Pierburg it isn't over pressure. Did you pneumatically sync the carbs?

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

      
    Report
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