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I was doing a carb sync on my 912 ULS and had the idle set at 2000 rpm while doing the sync, with my CrabMate hooked up per option 2 in the maintenance manual.  A couple minutes into the sync the engine started running very rough and fuel began overflowing out of the vent tubes on both carbs.  When I raised the rpm to 3000 it stopped on one carb vent tube but not the other so I shut it down.  I've read elsewhere that this can be caused by too high fuel pressure but what would cause that to happen suddenly?  I currently don't have a fuel pressure monitoring port in the fuel pump output line but I could add one if this is the most likely cause of the issue.  After all this I rewatched the Rotax carb sync video and they did the sync starting at 2500 rpm, which I'll try tomorrow.

  • Re: Fuel overflow from carb vent tube

    by » 4 months ago


    Hi Larry, did you remove the float bowl(s) from the carbs?  I took mine off once to drain old fuel.  When i put it back on something didn't seat correctly and the floats must have stuck in an open position.  When this happened, the fuel overflowed. 

    I know Rotax had an SB out for some of the floats.  I think some were bad (which caused some of them to stick - get soaked with fuel causing them to sink instead of float).  I doubt yours would suddenly start doing this, but it would be a possibility.  In my case, the service bulletin doesn't apply, but I did see what happens when the floats don't "float".


  • Re: Fuel overflow from carb vent tube

    by » 4 months ago


    I would look at:

    1.If this has only started since doing the sync then start by looking at what you have just done. Have you introduced an air leak? If the engine runs rough due to an air leak, then the shaking can cause the floats to sink causing the bowl to overflow.

    2. Are the floats too heavy? (Unlikely if the engine has been OK up to this point).

    3. Are the float valves closing off properly? Any debris on the seats?

    4. Has the fuel pressure increased to the point where the float valves are bypassed? It shouldn't be above 0.4 bar but it will cope with a bit more.


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