Prior to first flight I had been having some difficulty with carb vibration and subsequent fuel venting and one of the things I had tried to reduce fuel venting was to readjust the float level settings by bending the armature tab that the float valve interfaces with.  At the time that adjustment caused more closing force on the float valve and that did reduce the fuel venting, while maintaining an acceptable fuel level in the bowls, as evidenced by bowl removal after operation and measuring the fuel level in each bowl.  Since then I have done a successful P-Sync on the carbs and have eliminated the carb vibration and any fuel venting.  However, to date I have not readjusted the float valve armature back to the factory level setting.  

During my recent 1st flight the engine ran great at reduced power settings but would misfire for a few seconds on climb out at 5500rpm and above and then come back strong for a few seconds.  This cycle of missing, then coming back repeated until the power was pulled back during the downwind leg while in the traffic pattern.  With reduced power setting during descent the engine ran fine.  The fuel was checked for water contamination before and after flight and none was found.  The plugs were checked and were the normal tan color.  Ambient temp was 70F.

After the flight it occurred to me that the engine missing was most likely due to the float valve armature not being set to the recommended location.  I plan to return it to the proper setting before my next flight.  I have seen a 10 year old post in this forum that describes the float valve armature setting as 0.412" from the carb bowl "edge" to the the top of the metal float arm.  I am confused about where this "edge" dimension is taken from on the carb body and if it is still a valid dimension.  Can someone clarify this dimension for me?  Also, can someone confirm my suspicion that this float valve setting is the most likely cause of the engine missing condition I experienced in flight.  Thank you in advance for any insights you may have to share.

  • Re: Engine missing at 5500 rpm

    by » one month ago

    Take a look at the diagram in Jacques' post here:

    Jacques foucher fuel level in the carburetor bowl

    Mine measures 12.5mm and works well. If you have the wrong level set then that will affect the mid and top range fuelling. If you had to bend the armature to reduce fuel venting then that probably means that the fuel level is too low and that will probably lead to a lean mixture that may result in misfire at high power settings.

    If the carb sync is right, the floats are not sinking (have you measured their weight?) and the fuel level is correct in the float bowl then you should be good.

  • Re: Engine missing at 5500 rpm

    by » one month ago

    I did measure the float weight and they were all below the spec limit.  I have seen the post you referenced and when I measure the fuel level after an engine run it is the same as in the post (1/2" below the top edge of the bowl).  This is seen even though I appear to have the float arm tweaked to minimize a previous issue with fuel venting.  But the post you mentioned is a measurement of the fuel level and not the float arm setting from the carb body.  After I tweak the float arm back to the factory setting I'll reconfirm the fuel level in the bowls after an engine run to see where they are at then.

  • Re: Engine missing at 5500 rpm

    by » one month ago

    the correct setting for the floats arm is :

    when the carb upside down, the arm should be parallel to carb body

    the attached drawing shows for a 2 stroke, so the 10.5 mm (.412'') may not be valid but the principle is the same

    30847_2_222002F7-E1F1-4A41-8A12-785CBF88F91A.jpeg (You do not have access to download this file.)

  • Re: Engine missing at 5500 rpm

    by » one month ago

    Rotax-Owner.com - WOT miss when HOT    

    My story is a lot longer than what's at this link but your problem sounds close to what I had.

    Check your chokes and make sure they're assembled right.


  • Re: Engine missing at 5500 rpm

    by » one month ago

    OK guys, I asked for the level of bowls, because I don't ride Rotax floats.

    I am using Bing floats (two nylon floats tied together)

    I checked, I get 13mm with the bing floats for the BMW R100 motorcycles.

    I know, we must not ...🙄

    the floats cost me too much, and in my club, I have the experience of those who have tried for several years

    by no means am I saying that you must do like me.

    I just think I have to try, after my classic floats sank.

    In a year I will tell you my experience on this change.

    Jacques Foucher

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