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Thread: Carb sync? Never again.

  1. #21
    Senior Member Rodney's Avatar
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    Default Re: Carb sync? Never again.

    In doing some research on the subject of a balance tube I found the following on the Rotax Owners web site: An individual there wrote::

    ((Hope it's ok to post this: it's a probably a better explanation than I could write. I apologize if this is not proper etiquitte.))

    "Built and added a 1 1/2"D. balance tube to my 912S and it is remarkable how much difference it made in the smoothness at low RPMs. This is not new technology and has been used on intakes and exhaust systems since the forties to smooth out low end vibration. It works.

    I saw it advertised recently by an italian racing equipment firm called FLYGAS but they wanted something like $1200.00 for the package. (cost me about $100.00 to build). It does not increase HP, I don't know yet about fuel consumption. Its advantage is to reduce vibration and improve manifold airflow.

    Here's what happens. Firing order on the Rotax, and most other boxer engines, is such that when one cylinder is firing, the other cylinder on the same side is on a compression stroke which means that NO air is flowing into that manifold. This inrush of fuel/air mix which has suddenly stopped has nowhere to go except through the balance tube and into the other manifold which is calling for air. The Rotax balance tube is woefully small for this task, even though it is great for idling. You can see this starting and stopping of airflow in the wildly bouncing of the vacuum gauge needles during carb balancing. Because all of this is happening in fractions of seconds, most of the air in the balance tube never actually goes anywhere, but bounces back and forth in the large tube. "

    If this explanation is accurate, then the it appears that this modification is more a function of the style of the engine ( ie: boxer) rather than the displacement of the engine. The zipper kits only add to the cubic inches of the engine.

    Got a text from Hal and Jay this morning. Cost of their conversion is $300.00 plus shipping plus your old manifolds. I have no connection with Hal or Jay, am just posting this for the information of our group.
    Well, I did buy the 114HP Zipper Kit from Hal n Jay and I can report I'm very pleased with it.

    Seem like every source I read on this subject reports a major improvement in idling performance of the engine.

    Hope this sheds some light on the subject.

    Regards
    Rodney

  2. #22
    Senior Member Wheels's Avatar
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    Default Re: Carb sync? Never again.

    I like that explanation! it makes sense to me and I may be more inclined to make that mod ... after I fit the current engine to my plane. Yes, I know, I said the jury is usually out on a mod after about 5 years of field data, but, it sounds like we may be closer to about 40 years of data. I would like to do some historical research. ... after I fit my current engine to my plane. Did I say I was changing engines?

  3. #23
    Super Moderator Av8r3400's Avatar
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    Default Re: Carb sync? Never again.

    For the record my modification is to a 1" diameter balance tube.
    Av8r3400
    Kitfox Model IV
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    912UL 105hp Zipper
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  4. #24
    Senior Member
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    Default Re: Carb sync? Never again.

    Hi folks
    This is slightly off topic but pretty close. Today I was changing oil and decdied to check the carb balance on my 730 hour 912UL. It has been at least 4 years and 400 hours since doing this. Runs smooth but let's check.
    As soon as I connect the Carbmate the engine begins running rough and both carbs start flooding.
    The carbmate operates well when vacuum or pressure is applied.
    I t doesn't leak.
    Remove it and engine runs fine.
    I use a vacuum hose as a balance tube with a tee for manifold pressure gauge.
    It's been this way for many hours.
    Appreciate any suggestions
    Cheers
    Don

  5. #25
    Senior Member
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    Default Re: Carb sync? Never again.

    I would try a dual manifold pressure gauge or a different CarbMate.

  6. #26
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    Default Re: Carb sync? Never again.

    A Carbmate does not allow any air to pass back the forth like a balance tube does. That`s why after syncing the carbs the engine always runs smoother when the Carbmate is removed and the balance tube reconnected.

    The issue is that my engine does not even want to run unless the balance tube is installed!! What does this tell me?

  7. #27
    Super Moderator Av8r3400's Avatar
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    Default Re: Carb sync? Never again.

    To balance th carbs you must disconnect the balance tube and attach the two vacuume tubes to the manifolds and the carb mate.

    When you disconnect the balance tube you then see the actual balance of the now totally independent carb systems. Your carbs are out of sync, this is why the engine runs rough.

    The engine will alway roughen if the carbs are out of sync and no balance tube it present to equalize them. This is the essence of this modification, to allow free breathing between the manifolds.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
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  8. #28
    Senior Member
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    Default Re: Carb sync? Never again.

    It`s possible I did not explain the situation well.

    The engine runs great with the balance tube installed. And has for many years and hours. Smooth at all RPM`s.

    When I connect the Carbmate it floods and runs really rough. In fact it won`t even keep running.

    I wouldn`t think the carbs would be that far out of sync when it has been running so smoothly all along.

    You may be right, though! I will start from scratch with a manual sync and adjust it from there.

    Thanks for the suggestion.

    Cheers
    Don

  9. #29
    Senior Member Norm's Avatar
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    Default Re: Carb sync? Never again.

    Quote Originally Posted by kitfox2009 View Post
    I will start from scratch with a manual sync and adjust it from there.

    Thanks for the suggestion.

    Cheers
    Don
    Don I have been in your plane and agree it runs smooth at all rpms. I would be hesitant to start from scratch with a manual sync. Check with the vacuum gauges first but when it runs as well as it does proceed with caution.
    My 2cents worth.
    Wait Canadian got rid of the penny.
    My nickels worth
    Norm
    Airdrie Ab, Can
    North of Calgary
    Flying SuperFox Model IV

  10. #30
    Senior Member Slyfox's Avatar
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    Default Re: Carb sync? Never again.

    hook up the carb mates to the fittings in front of the carbs. you take out the little screws and hook up there. take the balance tube and block both ends. I used a straight hose between the two, when I balance I squeeze the hose off. if blocking the two manifolds yields a rough engine then the two sides are not close, need to go full throttle and check adjustment to the carb stops. then try again. if it's from hooking up the guage then go where I suggested and try again.
    steve
    slyfox
    model IV 1200-flying
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