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Thread: Carb sync? Never again.

  1. #1
    Super Moderator Av8r3400's Avatar
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    Default Carb sync? Never again.

    This past Airventure I had the opportunity to talk with Hal Stockman (the Zipper big bore guy) and he shared with me his latest modification to the 912 engines.


    He is removing the small balance tube between the intake manifolds and replacing it with a 1" diameter tube.

    I just did this modification to my engine this past weekend and had the chance to test run the engine today. All I can say is WOW! What a difference this makes. The engine is now butter smooth from 1500 to 6000 rpm. A mechanical sync is all I did on reassembly and don't plan to do any more. This mod is something I've been thinking about for a long while, but didn't have the courage to do without a second opinion. Makes perfect sense to me and makes me wonder why these engines don't come this way from the start.

    I forgot to snap a photo, but will do this tomorrow for the record.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  2. #2
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    Default Re: Carb sync? Never again.

    Yes, we or at least I would like more info and pictures. JImChuk

  3. #3
    Administrator DesertFox4's Avatar
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    Default Re: Carb sync? Never again.

    Very interesting developement. Standing by for photos.


    DesertFox4
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    912 ULS Tri-gear


  4. #4
    Senior Member Rodney's Avatar
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    Default Re: Carb sync? Never again.

    Did you have to do any machining to the intake manifold, or was this simply a bolt on addition?

    Thanks
    Rodney

  5. #5

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    Default Re: Carb sync? Never again.

    That sounds interesting.
    I am curious for the theory behind that modification.

  6. #6
    Super Moderator Av8r3400's Avatar
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    Default Re: Carb sync? Never again.

    I did some machine work on the manifolds to position and tap 3/4 NPT holes for the fittings. I got 3/4 NPT to 1" pipe 90 degree elbows from Jegs, 1" thin wall tube from a local supplier and 1" heater noose and clamps from NAPA.

    The theory behind the modification is to allow the two manifolds to equalize pressure or vacuume between them easier.



    Photos later today.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  7. #7
    Senior Member jdmcbean's Avatar
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    Default Re: Carb sync? Never again.

    Quote Originally Posted by Av8r3400 View Post
    This past Airventure I had the opportunity to talk with Hal Stockman (the Zipper big bore guy) and he shared with me his latest modification to the 912 engines.


    He is removing the small balance tube between the intake manifolds and replacing it with a 1" diameter tube.

    I just did this modification to my engine this past weekend and had the chance to test run the engine today. All I can say is WOW! What a difference this makes. The engine is now butter smooth from 1500 to 6000 rpm. A mechanical sync is all I did on reassembly and don't plan to do any more. This mod is something I've been thinking about for a long while, but didn't have the courage to do without a second opinion. Makes perfect sense to me and makes me wonder why these engines don't come this way from the start.

    I forgot to snap a photo, but will do this tomorrow for the record.
    I have heard of this many years ago through some overseas sources... never tried it but you are the 3rd person I have heard glowing reports from recently..
    John McBean
    www.kitfoxaircraft.com
    208.337.5111

    "The Sky is not the Limit... It's a Playground"

  8. #8
    Senior Member jiott's Avatar
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    Default Re: Carb sync? Never again.

    This is interesting to me too. I've been noodling on the theory and believe it makes perfect sense at the higher engine rpms because the intake flow is quite high and a bad carb balance may require a high flow thru the balance tube. However, down near idle the flows are very low and an imbalance would not need a very big balance tube to correct it, so I would think that the 1" balance tube would have very little value at near idle speeds. Thru the years of doing carb syncs I have noticed that if there is an imbalance it seems to always show up as far worse down in the very low rpms, but as you run the rpm up the imbalance seems to get better. I'm not sure how to explain this except that a small motion of the throttle cable near idle has a much higher percentage change of throttle opening than it does near WOT. Also, the "starting carb" circuit does not become inactive until rpms are higher. What this noodling is leading me to believe is that even if you go with a large balance tube, you should still occasionally do a carb sync at idle. Be interested in other opinions on this.

    One more thing, I don't think there is much value in installing a balance tube that has an inside diameter any larger that the smallest hole thru the fittings on each end.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  9. #9
    Senior Member Slyfox's Avatar
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    Default Re: Carb sync? Never again.

    Larry,
    you and I talked about this a OSH and it makes total sense. Now you need to go through how you did it. I can get the parts, but I be scared to drill out holes. right now I be busy putting on tires and brakes and all that good stuff. this one will be my next thing.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  10. #10
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    Default Re: Carb sync? Never again.

    I ordered my kit from Hal (ph: 775-934-5714) today. Worst case, if it doesn't pan out, you make an adapter to go back to original design. Hal's kit consists of modified manifolds, the 3/4 pipe thread fittings, new crossover tube and all required parts for installation. When done, you send him your old manifolds. This may help with a rumbling kind of vibration when throttling back to land (3300 down to 2500 rpm) at airspeed above 90 mph. All in all, this is a fun airplane.

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