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I have a carb that keeps on overflowing. I bought 2 x carb overhaul kits and replaced all components in the carbs. Floats, needle, seat, diaphragm etc etc. I also cleaned the carbs properly. Looking at the engine from the prop side, it is the carb on the left that is dripping fuel, generously.

On starting all if fine, but the carb that was overflowing, is still doing exactly the same thing.

The compensation aluminium pipe between the carbs, has holes in it, so I replaced it with a normal rubber tube, can this be the cause? I had recommendations that the engine mountings might be warn, causing the engine to shake, thus overflowing the carbs, this is not the case. Engine is stable and smooth.

Any ideas? Recommendations? Advise? I hit a brick wall :pinch:

wvbosch@gmail.com
  • Re: Carb Overflow

    by » 10 years ago


    Hi Willem,

    The engine shaking can cause this, but since you said yours was smooth we can move on. Just a side note** sometimes shaking to one is smooth to another. It can be subjective to some. I have had clients come in telling me how smooth the engine was running and the vacuum on the carb sync gauges was 10" off.

    I take it you have already done a carb sync?
    When you did the carb rebuild did you set the float armature height? If this is off then the floats can not regulate the fuel. If you did this then it's time to pull that carb back off because even though you think everything is okay inside something isn't quite right inside.

    My guess is it is the needle valve isn't in right or the float armature is the wrong height.

    Here is a few other things to check. Make sure the crush washers are in the right place where the fuel feed banjo bolt attaches to the carb. Make sure the float bowl is sitting squarely down in the grooved edge on the carb body. Make sure you didn't cut or twist the bowl gasket. Make sure the carb vent tubes are getting the same pressure by matching locations if you have something other than a Rotax airbox.

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


  • Re: Carb Overflow

    by » 10 years ago


    914 or 912?
    Overtightened float chamber screw (on the bottom) of the 914 can cause the float guide pins to spread out and stop the floats rising to their correct height. It happened to me once.
    Mike G

  • Re: Carb Overflow

    by » 10 years ago


    Thank you for the input. It is a 80HP Rotax 912UL machine.

    I will try all the recommendations. Roger, isn't the default configuration on the float arms correct? I don't really want to bend the float bracket. The mechanical sync is done, meaning the throttle bodies moves up at exactly the same time if you apply throttle. I also made no cable changes. I do however need to take her in for the pneumatic sync to be done.

  • Re: Carb Overflow

    by » 10 years ago


    Hi Willem,

    The pneumatic sync is more important than the mechanical sync. The mechanical sync only gets you in the ballpark at low rpms.
    If the pneumatic sync was off far enough which would make the engine shake and keep the floats from controlling the bowl fuel level then this could be the problem. Any time you rebuild a set of carbs the float armature height or level must be checkd. The part that gets bent if an adjustment needs to be done is the tab that the needle valve hooks onto. You should always check both sides of the armature to. You need to make sure they are equal and one didn't get tweaked dire some maint. If the float armature levels off syncing the carbs won't fix it.

    If it was me I would pull off the offending carb and check the float arm heigh then sync the carbs.

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


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