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Thread: Musings on vacuum leaks

  1. #1
    Registered User STEVENRANKIN's Avatar
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    Musings on vacuum leaks

    How many places in an airhead intake system produce vacuum leaks?

    This is a question I have yet to answer and the total is five places so far.

    Let me give you the places and see if you have had the same thing happen.

    One, the plugs for the air injection system removal in the head. backed out or got loose.

    Two, the intake metal tube in one head (where the carb mounts with a rubber tube). Forgive the technical jargon, LOL

    Three, the plugs for the air injection removal into the bottoms of the carbs.

    Four, the rubber tube holing the carb to the head clamps were loose on one side (the opposite side of the loose metal tube)

    Five, the side of the carb, looks like a plug in the side from the factory.

    Oh well, with 190000 miles on them I guess things come loose. At least they are easily fixable.

    It is surprising how small a leak can make such a difference in idle. St.

  2. #2
    Liaison 20774's Avatar
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    What about a leak in the plug on the dome top of the carb? While it doesn't affect the air mixture by direct injection into the flow, it prevents the slide from operating correctly, thus changing the air-fuel mixture as it goes through the carb venture.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Registered User STEVENRANKIN's Avatar
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    Opps! I forgot that one

    Opps! I forgot about the plug on top!

    That makes six.

    Thanks for reminding me of it. LOL. St.

  4. #4
    Throttle plate shaft O rings
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

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    Registered User ebeeby's Avatar
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    Quote Originally Posted by PGlaves View Post
    Throttle plate shaft O rings
    +1.....
    1973 R75/5

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    Registered User STEVENRANKIN's Avatar
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    So far

    So far, I have found and repaired all the leaks mentioned here except the throttle shaft O rings. Put new O rings in and they still leak.

    Perhaps it is time for a newer set of carbs. LOL. St.


    I swapped out carbs from a bike with less mileage and my bike runs so much better. At least now I know the problems are with the carbs.

    I don't want to give the newer carbs back! LOL.

  7. #7
    Registered User STEVENRANKIN's Avatar
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    Final solution

    Well, I like it that two people mentioned throtle shaft o rings.

    I had replaced them however for some reason, there is still a leak at the shaft.

    Swapped out carbs from another bike, runs like new.

    Bought two carbs from Roland Robilie. Having them rebuilt (All the new bits from the malfunctioning carbs going to the replacements) and crossing my fingers

    Other than perhaps oversize o rings, I don't know how else to seal the shaft.

    I guess at 190000 miles they are worn out. St.

  8. #8
    Liaison 20774's Avatar
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    What are you doing to determine they leak? What's the test?
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  9. #9
    Registered User STEVENRANKIN's Avatar
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    Test

    Engine running after a long ride. Rough idle.

    Spray with carb cleaner, engine idle smooths out, carb cleaner evaprates is sucked in, idle gets rough.

    Spray with water, idle smooths out.

    Simple fast and works. St.

  10. #10
    Registered User lkchris's Avatar
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    Quote Originally Posted by STEVENRANKIN View Post
    How many places in an airhead intake system produce vacuum leaks?

    This is a question I have yet to answer and the total is five places so far.

    Let me give you the places and see if you have had the same thing happen.

    One, the plugs for the air injection system removal in the head. backed out or got loose.
    This is not a vacuum leak.

    A vacuum leak is by definition something in the intake tract of an engine and occurs on the intake stroke ... which is what creates vacuum. A carb or fuel injection meters the air being taken in and provides the proper amount of fuel ... a vacuum leak is unmetered air.

    What you've described is simply a hole in the exhaust system, no different than a hole in a header or muffler.
    Last edited by lkchris; 06-10-2019 at 05:26 PM.
    Kent Christensen
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    '12 R1200RT, '02 R1100S, '84 R80G/S

  11. #11
    Registered User lkchris's Avatar
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    Quote Originally Posted by STEVENRANKIN View Post
    Three, the plugs for the air injection removal into the bottoms of the carbs.
    This is not a vacuum leak either ... it's the same thing as not having a filter in your air box.

    A vacuum leak by definition only occurs *between* the carburetor and the intake valve.

    Therefore, it's only created by carburetor components or intake manifold components.

    PS: you forgot the water drain in the bottom of the airbox ... it has to be there and remain flexible enough to close under vacuum.
    Kent Christensen
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    '12 R1200RT, '02 R1100S, '84 R80G/S

  12. #12
    Registered User STEVENRANKIN's Avatar
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    I am sorry

    I am sorry, I really do not understand the last post? Have you not been reading my posts? I am not talking about exhaust leaks or airboxes.

    I am talking about something that sucked in carb cleaner and ran smoother. ON THE AIR INTAKE Side.

    The plug in the head for the air injection system was loose, it sucked in carb cleaner and ran smoother. How can this be an exhaust leak? Exhaust is out intake is in.

    Perhaps vacuum leak is the wrong term, suction leak maybe?

    All this is happening with carbs and intake, not exhaust. St.
    Last edited by STEVENRANKIN; 06-10-2019 at 10:03 PM.

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