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Thread: 912 uls carb vent lines

  1. #1

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    Default 912 uls carb vent lines

    hi guys, reading the rotax installation manual it states:
    " The carburetor float chamber venting lines have to berouted into a ram-air and vacuum free zone ... These lines must not be routed into the slipstream orany other location that is subject to ram-air or vacuumduring flight or ground operations. "
    So what solutions are available - besides directly into the cockpit? :-)
    thanks in advance...Karl

  2. #2
    Senior Member jiott's Avatar
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    Default Re: 912 uls carb vent lines

    I routed mine into the back side of the air filters, because thats where they go if you have the Rotax supplied airbox. Been that way for 6 1/2 years.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  3. #3
    Senior Member jrevens's Avatar
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    Default Re: 912 uls carb vent lines

    I routed mine into the air filters also... it seems to work very well.

    IMG_2187.jpg
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  4. #4
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    Default Re: 912 uls carb vent lines

    Routing the vent lines to the air filters would definitely not be a "vacuum free zone." One of my buddy's ran his out the bottom into the slip stream (creating a vacuum) and it caused his engine to run rough at midrange. It's a catch 22 because a rough running engine will puke gas out of the vent lines. Mine are about 4" long and my engine is super smooth. My buddy did his like mine and now his engine runs as smooth as mine. A smooth running engine doesn't puke gas.

  5. #5

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    Default Re: 912 uls carb vent lines

    As an example, on the RV-12 with Rotax 912 ULS engine the carb vent lines are about 4" long and the ends are tucked down and under the right-side bail wire that secures the carb float bowl. This is a pretty common configuration for engine installations that have simple K&N cone filters (instead of the more complex and costly Rotax-style aluminum air box).

  6. #6
    Senior Member jiott's Avatar
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    Default Re: 912 uls carb vent lines

    When I first started my brand new 912uls the carb vent lines were tucked under the float bowl bale wires as suggested. It ran great for about a year that way. Then I decided to route them to the back of the air filters and the engine ran just the same as before, very smooth. They have been there ever since because it just makes sense to me; its just like the Rotax airbox, and if any fuel vents out the tubes it will go right back into the carbs to be burned instead of dripping down on the hot exhaust manifold. I understand that there is a slight vacuum inside the filters, but if you keep your filters clean it is very slight. It also should give good warning if fuel starts venting because the engine will start running too rich and will be rough. Anyway that's my thinking on it.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  7. #7
    Senior Member Dusty's Avatar
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    Default Re: 912 uls carb vent lines

    My vent lines run to the large style K N filters, when one vent line came off I only noticed during an inspection. There was no noticeable difference on the egt guage.

  8. #8
    Senior Member jrevens's Avatar
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    Default Re: 912 uls carb vent lines

    Quote Originally Posted by jrthomas View Post
    Routing the vent lines to the air filters would definitely not be a "vacuum free zone." One of my buddy's ran his out the bottom into the slip stream (creating a vacuum) and it caused his engine to run rough at midrange. It's a catch 22 because a rough running engine will puke gas out of the vent lines. Mine are about 4" long and my engine is super smooth. My buddy did his like mine and now his engine runs as smooth as mine. A smooth running engine doesn't puke gas.
    Depends on how much restriction there is in the filter, and if the needed airflow is greater than the filter can accommodate. I think the situation is very similar in performance to when the Rotax airbox is used. If you're going to leave them as received & recommended by Rotax, you'd be well advised to have some kind of drip pan underneath like the neat ones Lowell (Highwing) made, or the ones used on the RV-12s. That can make the removal of the float bowls much more of a pain in the neck though. I can see why your buddy's ran rough, with long lines going out into the slipstream (exactly what Rotax tells you not to do). The end of the cone filters that I installed the little elbows in is pretty thick and rubbery. The fittings fit in the hole tightly and just barely poke through on the inside. Like Jim said, and many others have also discovered, it works perfectly there, and I can tell you that my engine runs very smoothly throughout its range. As they say, "your mileage may vary".
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  9. #9
    Senior Member HighWing's Avatar
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    Default Re: 912 uls carb vent lines

    A picture of my vent line. This set-up was to eliminate the possible risk of a fuel drip onto the muffler as a couple of guys had reported. It provided a drain without moving the vent to a non neutral location.
    Attached Images Attached Images
    Lowell Fitt
    Goodyear, AZ


    My You Tube Channel

  10. #10

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    Default Re: 912 uls carb vent lines

    thanks everyone, all your inputs are truly appreciated - this is a very friendly community.

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