• Re: Carb Float Bowl Breather/Overflow

    by » 10 months ago

    Hi Sean,

    If you're  going to try and replace the seats anyway why not try and polish them? If it works problem solved.

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

  • Re: Carb Float Bowl Breather/Overflow

    by » 6 months ago

    Up Date!

    Sonex is back in the air.

    Carburettors have been installed (with new float valve seats) and balanced. Great performance - engine running like a sewing machine.


    In the first hour, I collected aproximately 100 + ml of fuel from the carb breather/vent tubes.

    In the next 2 hrs collected about the same - so less fuel coming through.

    The last 1 hr, only about 25 ml

    I hope the sealing progress will continue to ZERO fuel.


    From another Forum conversation: - it has been suggested, that should the float bowl fuel "boil" the floats would sink and thus more fuel would enter and possibly exit out the breather line. If this were to happen it would be most likely post flight/after engine shut down.

    I have gone to a fair bit of effort to reduce, by shielding, the potential for the carburettor float bowls to be affected by radiant heat from the engine/rear exhaust system - I can't do much about convection heat on engine shutdown.  (In flight the undercowl temperature is 30C  Ambient: 22C@ 6500 f, .  I have seen 50C + on the ground, post shut down).

    Bear in mind that my aircraft is a "taildragger" so after engine shutdown, I would expect the air flow to readilly reverse ie ambient/cooler air enter through the cowl bottom vent and hot air exit through the forward ( upper) "nostrils". I would hope the effect of this would be to see fairly rapid cooling of the engine/exhaust hopefully minimising convective heating of the carburettors.


    1. Anyone else had / having a similar issue?

    2. Do you have an opinion on fuel "boiling" in the carb float chamber?

    3. Any further suggestions?

  • Re: Carb Float Bowl Breather/Overflow

    by » 6 months ago

    Just get the Rotax floats with the "R" on the side.  The Bing "B" floats are not dependable.  


    38295_2_SB-912-074UL_SB-914-056UL_2ST-004_Exchange of floats pair on ROTAX Engine Types 912 914 and 2-ST Series.pdf (You do not have access to download this file.)
    38295_2_SB-912-074_SB-914-056_Exchange of floats pair on ROTAX Engine Types 912 and 914 Series.pdf (You do not have access to download this file.)

  • Re: Carb Float Bowl Breather/Overflow

    by » 6 months ago

    Hi RW -

    Have already replaced my OM  floats, within the last 12 months, supplied by Rotax dealership, so assumed to be Rotax parts.

    The new floats did not change the flow of fuel through the breather.

    Since then the 50 hr carburettors have been back to the dealership for the above mentioned inspection & remedial work. 

    Now have about 55 hrs on engine.

  • Re: Carb Float Bowl Breather/Overflow

    by » 6 months ago


    I would do the test as shown in the MMH on the float needle to seat check.  Be sure that they are sealing correctly.  While your at it confirm they sold you the R floats and not some of the Bing ones.  There are also several versions of the float needle, check the plunger color on the spring, it should be yellow.  (not silver)  There were 3 versions made, silver, the lightest.  There was a black version, the heaviest internal spring.  And the version we should currently use is gold (very light yellow).  This spring is a med tension and least likely to bounce with the 11 to 1 compression pulses on the 100 HP version engines.  Spare parts are always suspect depending on who supplied them.  Bing motorcycle parts are not necessarily correct in out applications.  


    Thank you said by: Sean Griffin

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