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Thread: Carb sync? Never again.

  1. #11
    Super Moderator Av8r3400's Avatar
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    Default Re: Carb sync? Never again.

    Here are the promised photos. I plan to get some better fitting clamps for the connector hoses. This is the proof of concept version.

    Crossover.jpg

    Crossover2.jpg

    Crossover3.jpg
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  2. #12
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    Default Re: Carb sync? Never again.

    Is that smaller hose for a manifold pressure gauge? Looks pretty simple to do, might just have to do it!

  3. #13
    Senior Member jiott's Avatar
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    Default Re: Carb sync? Never again.

    For those of you considering doing this, I would highly recommend looking thru the postings on this subject on Rotax-Owner.com. Click on the forums and then use the search feature, using Balance Tube as the keyword and Titles only. It will bring up about 80 postings which is interesting reading. What I got from it is:
    - Results seemed to be somewhat mixed, with mostly good results, mainly at the lower rpms.
    - There were several ways of hooking up the larger balance tube and fittings-some with bad results.
    - Most of those with good results seemed to find that they needed to limit throttle opening to 95% or else the engine would start to miss at further opening; nobody seemed to know why this is.
    - There was some concern about the intake tuning with a much larger intake system volume (can have a large effect on race cars).
    - Most agreed that with perfect carb sync there is no need at all for a balance tube of any size.
    - No one claimed that the larger tube would eliminate all carb sync forever, it just makes it easier and more forgiving of inaccuracies. A sync at or near idle still seemed to be a good thing to do.
    - Most seemed to be happy overall; a few were removing the larger tube and going back to standard.
    - There is an Italian company, I think, that was offering a kit to do this. I believe they use a spacer block in front of each carb with ports in it for the larger balance tube. It moved the carbs back an inch or two I believe.

    Anyway, interesting reading; people have been experimenting with this for a number of years.

    Av8r3400, curious if you have noticed missing/stumbling at 95%+ throttle opening?
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #14
    Senior Member Wheels's Avatar
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    Default Re: Carb sync? Never again.

    I have been watching this with some interest. The issues with carb sync are numerous, and mostly induced by the mechanics omission or error. Sorry, but I'm the mechanic most likely to be "That" guy. A sticky cable or poorly routed cable, a cracked or worn carb boot, a sinking float, a mis-adjusted float lever arm, a worn screw, a dirty anything etc. etc. etc. and you have a rough running Rotax. Stay clean, stay tuned, know your carb theory and fly with ease. I always say if it isn't stock, its wrong, but ... I see the Zipper kit guys are having a ball! the jury is out after about 5 years of testing. I"m not into wrenching like I was when I first started and frankly I just want to go flying. The constant tinkering with my motor and worry about its reliability just seems to increase as I add any variation to what the original engineer intentioned. And Here I am modifying the holy Cr@#p out of a motor so that I can make it fit in my plane. The only difference is, I"m not trying to improve performance, I'm just trying to make it fit and run in the parameters for which it was designed. Seems like airplanes just BEG to be worked on.

  5. #15
    Super Moderator Av8r3400's Avatar
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    Default Re: Carb sync? Never again.

    Yes the small line is for a manifold pressure sensor.

    I had the opportunity to put an hour of touch and goes on this afternoon. The results are in: The modification works wonderful. The engine is smooth through all of the rpm range. My nagging rough spot from 2500-3500 is gone.

    Full throttle seemed to yield about 100 rpm more at static, with no roughness or missing. There are a lot of nervous nellies on fly-Rotax that refuse to believe anything but the party line from Rotax. I like my engine. A lot. I do believe there are areas that can be improved. This is one of them.

    I had synced and synced my carbs with analog and digital manometers and could never get the midrange roughness to go away. Now it's gone. Smooth idle to 1400 with no chatter.


    Very happy with the mod. No regrets.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  6. #16
    Senior Member efwd's Avatar
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    Default Re: Carb sync? Never again.

    Nice, good to here things are working smoothly.

  7. #17
    Senior Member Slyfox's Avatar
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    Default Re: Carb sync? Never again.

    I heard you could now go down in your idle without vibration, like 1200 with no problems.
    steve
    slyfox
    model IV 1200-flying
    912uls
    IVO medium in-flight
    RV7A-flying
    IO-360
    constant speed prop

  8. #18
    Senior Member Norm's Avatar
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    Default Re: Carb sync? Never again.

    Quote Originally Posted by Av8r3400 View Post
    I like my engine. A lot. I do believe there are areas that can be improved. This is one of them.

    Very happy with the mod. No regrets.
    Keep in mind this mod is with a Zipper kit. Other results may vary.
    I had seen this mod talked about on other forums and had considered doing this to mine before this thread showed up. Sounds like this is more fuel to add to the should do list.
    I would also love to do the Zipper mod but dollars and cents don't allow at this time. Thanks for the report.
    Norm
    Airdrie Ab, Can
    North of Calgary
    Flying SuperFox Model IV

  9. #19
    Senior Member
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    Default Re: Carb sync? Never again.

    Quote Originally Posted by Av8r3400 View Post
    Yes the small line is for a manifold pressure sensor.

    I had synced and synced my carbs with analog and digital manometers and could never get the midrange roughness to go away. Now it's gone. Smooth idle to 1400 with no chatter.


    Very happy with the mod. No regrets.
    Thank you sir! This is also the RPM range that I can't seem to get the 'shake' out. The roughness in that RPM, especially when I am over all the trees around my strip, is never comforting. If I can relieve this I would be a much more ardent Rotax supporter. It just bothers the bejeesus out of me when I am landing and the motor is rough.

  10. #20
    Senior Member jiott's Avatar
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    Default Re: Carb sync? Never again.

    This midrange roughness at 2500-3500 rpm that many of you talk about has always puzzled me. Most of you are experienced Rotax owners with many more hours than I, so I don't doubt for a minute what you are saying. What puzzles me is I don't ever see any sign of it on my 912uls with 500 hours on it now, and there are others out there with my same experience. Being a meticulous engineer type, I carefully synced the carbs mechanically and pneumatically from day 1 and have never had a roughness problem. In fact my sync hardly ever seems to move or get out of adjustment-the last time I needed to readjust slightly was 300 hours ago; it has stayed as perfect as I can get it for 300 hours without touching it.

    I'm not at all trying to imply that those with this problem are not doing the sync carefully enough; there must be something else going on that my engine fortunately does not have. The fact that your roughness shows up in a narrow range of rpms seems to eliminate things like prop balance or a partially sinking carb float. Could it be your throttle cable is not smooth thru midrange? Have you totally removed the throttle springs so that the cable does not have a consistent tension thru the stroke? I am using the lightest McFarland springs.

    I believe at these rpms is where the carb idle circuit makes the switch to the main circuit; could it be some deposits are causing the switchover to be rough?

    The large balance tube seems to solve whatever the underlying cause is and that is a good solution and it also seems to give you a smoother idle at lower than normal idle speeds. The lowest I can get a smooth idle is about 1600 rpm.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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