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Thread: Gremlins working OT. Bizarre chain of events leading to black fur plug.

  1. #1
    FORUM SUPPORTER thunderkat59's Avatar
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    Unhappy Gremlins working OT. Bizarre chain of events leading to black fur plug.

    I posted this thread about 3 weeks ago:
    http://www.sr500forum.com/vbforum/up...ystem-question

    Since then, Ive cleaned the carb and gotten the bike running well. Ive ridden it more than a few times.
    Last week, I get a pinhole in an oil line going from the tank to the head that was dripping oil on the fins and pipe.
    I fix that issue. Now, for some reason, the bike bogs and sputters off of a nice idle (1200 rpm). When I pull the plug, it is so black it looks
    like black moss is growing the porcelain! The carb could get that out of wack so soon after cleaning and adjust that worked?
    Any opinions on how these two seemingly unrelated issues might be related? Or--is it just fun and hilarity of owning on a 40 year old bike?
    There is no explanation.

  2. #2
    Senior Member MadMax's Avatar
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    It's mid-life crisis time for your carb, assuming the std VM34 is still there, it sounds very much like one or other of the diaphragms has finally collapsed, speak to Mark @ Thumperstuff (by phone), he has what you need.

    Max

  3. #3
    Senior Member gcrank1's Avatar
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    If you are still running the vacuum petcock pop the line at the carb/intake and see if any gas dribbles out. If so, the petcock diaphragm is holed and when ever the running vacuum is high it sucks gas through the petcock enrichening the mix completely separate of the carb.
    If you do not see any gas dribble out of that vac line it does not mean that the diaphragm is good, it can be bad as described it is just that you had no residual gas in that line to dribble; its just that the dribble is a quick definitive test.
    And yes, it is just coincidence, totally unrelated.
    '81 Yamaha SR500 CSR (Coffee Shop Racer) 'Work in Progress'
    '09 Suzuki TU250X 'The Silent Red Fellow'
    In all things maintain your uprightness lest the road reach up and smite thee

  4. #4
    FORUM SUPPORTER thunderkat59's Avatar
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    Quote Originally Posted by MadMax View Post
    It's mid-life crisis time for your carb, assuming the std VM34 is still there, it sounds very much like one or other of the diaphragms has finally collapsed, speak to Mark @ Thumperstuff (by phone), he has what you need.

    Max
    I thought only one diaphragm? The accelerator pump. But, thank you very much Max. really appreciate that.
    It is the stock carb, so I will email Mark tonight. I ordered new fuel lines and filter, too. I get it to start and idle
    as normal, but as soon as I get on the gas to ride, it runs exactly like the choke is on, but the choke lever is up
    as it should be. Just a run around the block makes the plug look like its covered in black moss, very thick.
    From totally clean to black fur in one block.
    It seems like as soon as I get this bike running like a normal bike after every issue that arises. Another one comes
    up. Very frustrating.

    Thanks again, Max.

  5. #5
    FORUM SUPPORTER thunderkat59's Avatar
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    Quote Originally Posted by gcrank1 View Post
    If you are still running the vacuum petcock pop the line at the carb/intake and see if any gas dribbles out. If so, the petcock diaphragm is holed and when ever the running vacuum is high it sucks gas through the petcock enrichening the mix completely separate of the carb.
    If you do not see any gas dribble out of that vac line it does not mean that the diaphragm is good, it can be bad as described it is just that you had no residual gas in that line to dribble; its just that the dribble is a quick definitive test.
    And yes, it is just coincidence, totally unrelated.
    Thank you gcrank . . .
    I had the tank on and off a few times over the course of the weekend fixing the oil leak.
    I wonder if I couldnt have damaged something in all of the on-and-off'ing?
    Im going to order the petcock diaphragm anyway. Glad I saw this right away.
    I would have never been able to make a diagnose like that by myself.
    I appreciate the help.

  6. #6
    Senior Member gcrank1's Avatar
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    And you do not have to remove the petcock to do this if you have a min. amount of fuel in the tank (down to needing Reserve or less).
    Just pop the lines, pull the tank and prop it carefully so gas doesnt run out the cap and is below the petcock. I usually throw an old shipping blanket over a car tire on the floor, the center hole lets me position the tank and it cant fall out/off.
    Then pop the screws on the petcock backside to get at the bits. BEWARE, there is a small spring in there, DO NOT lose it, they are unobtainium (though I have an 'extra').
    '81 Yamaha SR500 CSR (Coffee Shop Racer) 'Work in Progress'
    '09 Suzuki TU250X 'The Silent Red Fellow'
    In all things maintain your uprightness lest the road reach up and smite thee

  7. #7
    FORUM SUPPORTER thunderkat59's Avatar
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    Quote Originally Posted by gcrank1 View Post
    BEWARE, there is a small spring in there, DO NOT lose it, they are unobtainium (though I have an 'extra').
    I love this forum!!!
    That info is invaluable! I -WILL- lose any spring or proprietary screw/widget on any bike in any work session!
    Having this info might break that trend, ha ha

  8. #8
    Senior Member chunk's Avatar
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    Mark @ thumper stuff is a great resource, but he's not always on top of his email. Phone call seems the best way to order suff, in my experience anyway. Cheers.

  9. #9
    Once you get the diaphragms fitted, return your idle adjust screw etc to it's correct number of turns out ( 1 1/2) just in case the P.o. has had a fiddle to compensate for dodgy diaphragms. Then you can work from there if your pipe air filter aren't stock.
    Simon.

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