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Thread: Vacuum leak?

  1. #11
    KEITH KRAUSE's Avatar
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    So you've eliminated a problem throttle cable as a possibility. You exercise the rack, it appears to move freely but the bike still sometimes hangs up at 4,000 rpm's.
    At 4,000 rpm's you should be able to notice the throttle valves opening. I wonder if one or more are hanging up? When the bike is hanging up, can you see any difference in the throttle valves opening in relation to each other? If you carefully tap each valve with a tool does it drop to normal rpm's? Have you serviced the plates or the mechanism?
    Let me give you a classic example of an air intake leak. A leak between the carb and cylinder head. Example: on a cold bike that has the idle set correctly, the bike starts with choke, warms up about 90 seconds, choke off, and then should hold an idle of around 900 to 1,000 rpm's. As it fully warms, it's normal for the rpm's to rise an additional 100-200 rpm's but that's all. With an intake leak, once fully warmed up, the rpm's will rise much higher than just 100-200 rpm's. So the unknowing owner manually adjusts the idle control to around 1,000 rpm's and hopes the problem went away. The bike is turned off and sits until cold. The owner later starts the bike with choke and as soon as the choke is closed the bike stalls from lack of rpm's, and won't idle until the idle is manually adjusted higher. Same scenario over and over. The intake leak isn't going to go away. While running and playing with the throttle, the intake leak isn't going to be there sometimes and sometimes not. That's my experience. One other intake leak area is the ports where you connect the hoses for a vacuum tool synch gauge.
    If your bike isn't doing the above, I doubt it's a leak. I think it's mechanical and involves a moving part in the carbs or? I know you said you checked the timing mechanism for proper return, but have you put a timing light on it to see if the timing is correct and reacts quickly and consistently to throttle/rpm changes?
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  2. #12
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    cowboyup3371 is offline Forum LongTimer Past Site Supporter
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    I’d get new boots as they should all just jump onto the carbs equally at the same time. You should also consider getting new clamps

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  3. #13
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    Quote Originally Posted by cowboyup3371 View Post
    I’d get new boots as they should all just jump onto the carbs equally at the same time. You should also consider getting new clamps
    New clamps already and I'm gonna end up doing a spraytest on the intake boots before I get new ones, and new intake boot rings.
    Ian

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  4. #14
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    Quote Originally Posted by KEITH KRAUSE View Post
    So you've eliminated a problem throttle cable as a possibility. You exercise the rack, it appears to move freely but the bike still sometimes hangs up at 4,000 rpm's.
    At 4,000 rpm's you should be able to notice the throttle valves opening. I wonder if one or more are hanging up? When the bike is hanging up, can you see any difference in the throttle valves opening in relation to each other? If you carefully tap each valve with a tool does it drop to normal rpm's? Have you serviced the plates or the mechanism?
    Let me give you a classic example of an air intake leak. A leak between the carb and cylinder head. Example: on a cold bike that has the idle set correctly, the bike starts with choke, warms up about 90 seconds, choke off, and then should hold an idle of around 900 to 1,000 rpm's. As it fully warms, it's normal for the rpm's to rise an additional 100-200 rpm's but that's all. With an intake leak, once fully warmed up, the rpm's will rise much higher than just 100-200 rpm's. So the unknowing owner manually adjusts the idle control to around 1,000 rpm's and hopes the problem went away. The bike is turned off and sits until cold. The owner later starts the bike with choke and as soon as the choke is closed the bike stalls from lack of rpm's, and won't idle until the idle is manually adjusted higher. Same scenario over and over. The intake leak isn't going to go away. While running and playing with the throttle, the intake leak isn't going to be there sometimes and sometimes not. That's my experience. One other intake leak area is the ports where you connect the hoses for a vacuum tool synch gauge.
    If your bike isn't doing the above, I doubt it's a leak. I think it's mechanical and involves a moving part in the carbs or? I know you said you checked the timing mechanism for proper return, but have you put a timing light on it to see if the timing is correct and reacts quickly and consistently to throttle/rpm changes?
    When i checked the timing way back when i changed the spark plug caps it sat st the right spot but went way past F when given throttle, but it would return correctly. Is my timing mechanism adjustable?
    Ian

    1982 GS650GLZ

    Want a "twin" for my bike

  5. #15
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    Quote Originally Posted by KEITH KRAUSE View Post
    So you've eliminated a problem throttle cable as a possibility. You exercise the rack, it appears to move freely but the bike still sometimes hangs up at 4,000 rpm's.
    At 4,000 rpm's you should be able to notice the throttle valves opening. I wonder if one or more are hanging up? When the bike is hanging up, can you see any difference in the throttle valves opening in relation to each other? If you carefully tap each valve with a tool does it drop to normal rpm's? Have you serviced the plates or the mechanism?
    Let me give you a classic example of an air intake leak. A leak between the carb and cylinder head. Example: on a cold bike that has the idle set correctly, the bike starts with choke, warms up about 90 seconds, choke off, and then should hold an idle of around 900 to 1,000 rpm's. As it fully warms, it's normal for the rpm's to rise an additional 100-200 rpm's but that's all. With an intake leak, once fully warmed up, the rpm's will rise much higher than just 100-200 rpm's. So the unknowing owner manually adjusts the idle control to around 1,000 rpm's and hopes the problem went away. The bike is turned off and sits until cold. The owner later starts the bike with choke and as soon as the choke is closed the bike stalls from lack of rpm's, and won't idle until the idle is manually adjusted higher. Same scenario over and over. The intake leak isn't going to go away. While running and playing with the throttle, the intake leak isn't going to be there sometimes and sometimes not. That's my experience. One other intake leak area is the ports where you connect the hoses for a vacuum tool synch gauge.
    If your bike isn't doing the above, I doubt it's a leak. I think it's mechanical and involves a moving part in the carbs or? I know you said you checked the timing mechanism for proper return, but have you put a timing light on it to see if the timing is correct and reacts quickly and consistently to throttle/rpm changes?
    Carbs were also rebuilt not that long ago so I'd assume theyd be clean, cant do anything until tomorrow involving the bike being on.
    Ian

    1982 GS650GLZ

    Want a "twin" for my bike

  6. #16
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    A quick tip to rule out any leaks at the intake boots and even the inlet valves if needed.
    With the carbs off , spin the motor for a second or so and place the palm of you hand over the intake as you do this.
    You will feel suction and as you do this momentarily release the starter and pull you hand away.
    You will hear a pop and also feel the amount of suction.
    Do this a few times across the cylinders in fairly quick succession and once you have the knack you be able to differentiate both audibly and by feel if one or more is not holding the depression or not pulling like the rest.
    Finally delay pulling your hand off each inlet and you will feel how quickly each one loses suction and it will be obvious if one is normalising quicker than the rest.
    This test puts way more suction on the boot joint etc than would ever be seen in normal use and if you can't detect a significant difference between them doing this there is no way there is a leak there under normal running conditions.

    Also a static test for the diaphragms and carb top leaks.
    Simply fully lift two slides at a time with your index and middle finger and let them drop.
    Observe the speed at which they both close which should be at equal rates, smooth and damped.
    Do this across all four to compare all equally.
    If one falls rapidly or snaps shut you have a diaphragm split or leaking carb top etc.
    If one falls much slower ( which is what you are really looking for ) then you possibly have some slide stiction or some other issue such as a bent jet needle binding in the jet etc.
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  7. #17
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    Nice! I like performing tests more than inspecting. I just gotta ask before I do this, would compression play a factor in this? Pretty sure all my cylinders are good so I shouldn't have to worry but I'm just curious. Need to buy a compression tester at some point.

    At this point I'd pay somebody on here to come and help me figure out what's going on, wish more people were in florida! I know it's all simple stuff but man, once you basically have worked on every section of the bike and you're still having issues, it hits you a bit.
    Ian

    1982 GS650GLZ

    Want a "twin" for my bike

  8. #18
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    If you have a really bad cylinder it will show but we are talking a very significant compression drop caused by severe exhaust valve leak or damaged bore to affect the suction test when spinning on the starter.
    With the comp test around 15psi difference between the highest and lowest readings is acceptable on the average modest compression of a road motor.
    Surely there's someone nearby who could have a look..
    Maybe putting feelers out on the forum may get some response from someone local.
    A couple of hours or so is all it should take to find the problem.
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  9. #19
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    Quote Originally Posted by zed1015 View Post
    If you have a really bad cylinder it will show but we are talking a very significant compression drop caused by severe exhaust valve leak or damaged bore to affect the suction test when spinning on the starter.
    With the comp test around 15psi difference between the highest and lowest readings is acceptable on the average modest compression of a road motor.
    Surely there's someone nearby who could have a look..
    Maybe putting feelers out on the forum may get some response from someone local.
    A couple of hours or so is all it should take to find the problem.
    Okay cool will probably be doing it tonight, if I cant seem to find anything out of the ordinary I might very well make a post asking if anyone could come by and help out. Not to mention the great roads there is here to ride.
    Ian

    1982 GS650GLZ

    Want a "twin" for my bike

  10. #20
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    You've gotten some good advice from Zed. When you do the spray test, along with the intake boots , spray the carb bodies as well. Especially around where the throttle shaft goes through the body. They can get worn and can be sucking air through there.

    Mad
    83 GS750E
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