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Thread: Bike carbs with servo vacuum

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    Spanner Monkey

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    Bike carbs with servo vacuum

    Ok so I've got zx9 carbs and I've decid d to use a servo. However as we know these are vacuum assisted so was just wondering how you guys have got around this ? Separate pump ? Drilledand tapped the manifold as its only really short. Thanks in advance

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    Re: Bike carbs with servo vacuum

    Anyone ?

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    Re: Bike carbs with servo vacuum

    Would have said a carb is a carb and all engines will create a vacumn behind a restriction be it a butterfly or slide. What I would suggest if you are concerned about it is pull a pipe off all inlet tracks to equal it out.

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    Re: Bike carbs with servo vacuum

    vacuum is a vacuum, length of manifold doesnt matter unless its so short you cannot physically fit a take off

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    Re: Bike carbs with servo vacuum

    A single inletrunner-vacuum will not be strong enough for a servo.
    Connecting all runners together will cause running issues for the engine.

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    Re: Bike carbs with servo vacuum

    Correct, very often, with faster cams you get problems linking different cylinders for stronger vacuum.

    Very thin tubes can sometimes cover the problem but still depend on your cam.

    The best solution is taking vacuum from 2 cylinders for a stronger signal and fit one anti reverse valve for every connection. Best to use cylinder 1 and 4 or 2 and 3. It is possible to use all 4 cylinders with 4 anti reverse valves but with the street cams we use it's usual good enough with 2.

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    Re: Bike carbs with servo vacuum

    Quote Originally Posted by Miniliteman View Post
    A single inletrunner-vacuum will not be strong enough for a servo.
    Connecting all runners together will cause running issues for the engine.
    i have to disagree there, ive literally seen 1000's of cars working just fine with vac from only one manifold runner, with an occasional car using two

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    Re: Bike carbs with servo vacuum

    Quote Originally Posted by Miniliteman View Post
    A single inletrunner-vacuum will not be strong enough for a servo.
    Connecting all runners together will cause running issues for the engine.
    How ? If all runners connected to same consumer of vacumn obviously with one way in each runner take off, they must draw the same amount of vacumn so equal to each carb ? Plus the way to balance carbs on their original fitment ...bikes are balanced using a vacumn gauge. So if you pull vacumn off one or two runners is going to upset one or two carbs respectively. So sinario of someone left foot braking with throttle ( less vacumn ) is in My mind going to play havoc with carbs if they are so critical to vacumn to operate correctly ?

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    Re: Bike carbs with servo vacuum

    Ford thought it was ok for the Lotuses!

    Click image for larger version Name:	s-l1000.jpg Views:	87 Size:	88.3 KB ID:	81300

    Yes they are Webers and not CV bikes carbs but they all respond to airflow / vacuum as part of their function.
    Last edited by katana; 22-07-2017 at 12:19.

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    Re: Bike carbs with servo vacuum

    There are many Alfa's and other cars only using 1 cylinder vacuum but they are also using slightly different servo brake actuator. Like the vacuum advance canister from and Alfa, also different setting as a "usual" vacuum canister.

    Many many VW Van drivers I've converted to carburators are using only one cylinder but you do feel you need to push a lot harder. It's working but no longer as it was with 4 cylinders (if original setup with 4). By using 2 cylinders it's already better but please note, once you got a serious cam the vacuum is already not as deep as with standard road cams. One ore reason to select your fast road cam very careful.

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    I'm lost. So is it better to connect off 2 inlets . Than all 4

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    Re: Bike carbs with servo vacuum

    Vacuum is funny stuff as its doesn't ACTUALLY exist - its the absence of air so you can lose any of it just reduce is effect. But as nature hates a vacuum it will equalise throughout whatever is connected very quickly - so 1, 2 or 4 connections will make not a lot of difference IMHO

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    Re: Bike carbs with servo vacuum

    Someone correct me if I'm wrong here but doesn't a servo store the vacuum for when you push the brakes?
    It's not constantly sucking air and only gets replenished after you have pushed the brakes and let go!
    So I would of thought a decent size take off on one Runner would be sufficient when the butterfly is closed to replace the bit of vacuum you've just used before you get back on the loud pedal again.

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    Re: Bike carbs with servo vacuum

    you are correct a servo does store the vacuum, in fact it should hold enough for several pedal applications even when the vacuum scores is no longer present, ie a stalled engine

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    Re: Bike carbs with servo vacuum

    If you want to add a pump for backup assistance to engine vacuum. An awful lot of modern cars already use one

    Just using this one as an example as it was first that popped up in a search, but they can be found for low £20ish. They draw around 4-5A when active.

    http://www.ebay.co.uk/itm/BRAKE-BOOS...IAAOSwaBNZjKiS

    But the exact same pump is fitted to VW, Mercedes, Volvo and many others.

    Volvo seem to use a nice simple 3 port vac switch, one port to intake, one to pump the other to the servo and the switch will turn off/on to energise a relay to maintain vacuum in the servo.

    Again, just one example

    http://www.ebay.co.uk/itm/2004-VOLVO...cAAOSw65xZpxzS

    It may be possible to use such a pump as the only source of vacuum for the brakes, but not sure if any of the cars they are fitted to would actually do that. Nor would I want to take that risk. But they may work ok for it.
    When it's so easy to use it as an assistance pump, not adding engine vac would seem silly.

    Also has the benefit of should the engine die, or you were towing the car etc and ignition powered on, your brakes would still work.
    9.85 @ 145mph 202mph standing mile

    https://www.youtube.com/watch?v=Ss_c7fML3rw

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    Re: Bike carbs with servo vacuum

    double post for some reason
    9.85 @ 145mph 202mph standing mile

    https://www.youtube.com/watch?v=Ss_c7fML3rw

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    Re: Bike carbs with servo vacuum

    It all depends on volume, if the volume of anything you're trying to draw a vacuum in is bigger then the longer it will take to pull the vacuum, the engine cylinders are effectively the vacuum pump, you could argue that a shorter manifold will actually pull a vacuum quicker due the reduced volume.

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