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Thread: 912 rotax shakes

  1. #1
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    Default 912 rotax shakes

    Hi guys, well I've been flying the Kitfox 4 that I finally finished, and have a question. My engine is a 912 80 HP with about 120 hours on it. When I pull power back when I enter the pattern, I get a fair amount of shaking between about 3000-4000 RPM. Smooth above and below those RPMs. Haven't noticed it as the engine spins up, but then I'm pushing in the throttle faster then I pull it back when entering the pattern. I have a 3 blade 72" IVO ultralight prop. I think I have read others mention this, but don't remember the possible reasons. Anyone care to offer an opinion? Thanks, JImChuk

  2. #2
    Super Moderator desertdave's Avatar
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    Default Re: 912 rotax shakes

    I had an issue like this. It turned out that I had a bad float in my carb and the motor would flood out when I pulled back the power. Random guess but worth looking into.
    Good luck!
    Dave
    KitFox 6 Taildragger
    912 ULS
    Whirlwind 70” Prop
    Garmin G3x
    All around nice guy

  3. #3
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    Default Re: 912 rotax shakes

    Thanks Dave, when I was fighting with the other 912, I noticed it had what seems like the same kind of shake at around those rpms. I never did fly that engine though, but noticed the shake when slowing down from a high rpm on the ground. I weighed all the floats from both engines and they were all lighter then the maximum. I think the heaviest one was 3.1, and at least one was 2.8 grams. My EGTs are pretty low on the engine now though, just monitoring 2, and not sure how accurate they are, but one is about 1050, the other one is around 1200. I'll look at the plugs and see how black they look. JImChuk

  4. #4
    Senior Member rv9ralph's Avatar
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    Default Re: 912 rotax shakes

    Another cause of this shake could be when the prop windmills, due to the gearing, the compressions come close together, causing the shakes. When decelerating, leave some throttle in to reduce the shakes. I had the same issue on my model 3 with a 912 when I first starting flying it.

    Ralph

  5. #5
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    Default Re: 912 rotax shakes

    Ralph, what kind of prop were you running? My IVO seems pretty flimsy. I do have a two blade warp 72" square tip, I might give a try at some point as well. Those blades are a lot stiffer then the IVO ones. Thanks for the reply as well. JImChuk

  6. #6
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    Default Re: 912 rotax shakes

    My 912ULS has more of a harmonic around that 3,500rpm range as well. Smooth above and below. Airmaster 75" but it did it with the IVO as well.

  7. #7
    Senior Member rv9ralph's Avatar
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    Default Re: 912 rotax shakes

    Ralph, what kind of prop were you running?
    At the time I had the original recommended GSC.

    Ralph

  8. #8
    Skipatrol's Avatar
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    Default Re: 912 rotax shakes

    Quote Originally Posted by avidflyer View Post
    Hi guys, well I've been flying the Kitfox 4 that I finally finished, and have a question. My engine is a 912 80 HP with about 120 hours on it. When I pull power back when I enter the pattern, I get a fair amount of shaking between about 3000-4000 RPM. Smooth above and below those RPMs. Haven't noticed it as the engine spins up, but then I'm pushing in the throttle faster then I pull it back when entering the pattern. I have a 3 blade 72" IVO ultralight prop. I think I have read others mention this, but don't remember the possible reasons. Anyone care to offer an opinion? Thanks, JImChuk
    I have the same prop but the 100hp ULS. Same vibration in the same range. I agree with the others that it is likely normal harmonic vibration. If something was seriously wrong with your setup I would assume vibration would occur at all RPMs. If it annoys me I adjust my RPMs.
    George
    Granite Bay, CA.
    Kitfox Light Squared
    912ULS Tri Gear

    My YouTube Channel

  9. #9
    Mcslamma's Avatar
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    Default Re: 912 rotax shakes

    Hey, Others have said increasing the size of the balance tube/fittings from the intake manifolds helps tremendously, also adding a balance master ring has been mentioned in the past, personally I have the 80hp with similar vibrations at the same time when pulling back the power, I did add an increased balance tube size, and it helped, also I added a balance master on prop hub, it helped some more. Of course the carbs need to be balanced and as has been mentioned the carb floats need to be the proper weight, and floating etc.. As well as the vent tubes on the carbs have an affect on things.. I am happy with the way mine runs now, but it has been an evolution to get there..lol

    Oh ya, mine I an IV prop 72" cut back to 70" iFA
    Last edited by Mcslamma; 12-02-2020 at 06:02 PM. Reason: added info

  10. #10
    Senior Member Dave S's Avatar
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    Default Re: 912 rotax shakes

    Regarding the floats, a quick and dirty evaluation can be done on a 912 by carefully pulling off the float bowl so as not to spill any fuel. The floats have the small rods sticking out of their sides to actuate the needle valve and these rods should be riding right at the fuel level. If a float is heavy enough to submerge the small rods below the fuel level - the float is probably out of spec heavy.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

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