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What might cause two petcocks to fail

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  • flyboy
    started a topic What might cause two petcocks to fail

    What might cause two petcocks to fail

    It feels like I just replaced my petcock yesterday (in all reality, it was last August). So when the bike started having issues starting up, that was the last thing on my mind.

    Yet my petcock seems to be leaking copious amounts of fuel into the vacuum line (evidence so far - vacuum line smells like fuel, crankcase oil level went considerably up, gasoline dripping into a puddle under the bike, and lastly - petcock shows fuel on the vacuum side of the membrane - see pic below).

    What could be causing two petcock valve membranes to fail in less than a year? I burn 94 octane Chevron (with Techron), and have only added fuel stabilizer for winterization during this winter. Could it just be coincidence wrapped in bad luck?

    Puzzled.


  • Kreylyn
    replied
    Originally posted by flyboy View Post
    It is indeed a different color - it's black. It's a hard one to notice on that photo so I'm highlighting it with red dashes here:


    I should probably add that I further investigated, made some interesting discoveries, came up with a theory, and am now wondering what you guys think of it:

    First of all, I found no holes in the membrane whatsoever. I reassembled the petcock, created negative pressure on the vacuum line, then sealed the vacuum end while depressurized to see if it would eventually equalize and arrest the fuel flow. It didn't - the valve appears to be working as expected. Yet I did find fuel on the vacuum side of the membrane, as evdent in the photo.

    So now I'm wondering if fuel never magically passed through the membrane, but instead condensated from the vapors that came up the vaccum line from the carbs.

    My theory is that perhaps carburetor is failing to achieve two things:

    1. Failure for vacuum to equlize immediately after the engine shuts off. That would keep the petcock valve open for a while after the shut-off, allowing fuel to flow to the carburetor.

    2. Failure of the carb to prevent fuel from flowing into the engine after it's shut off. That would allow fuel to a) seep around the rings into the crank case (hence the elevated fuel levels), b) cause gasoline vapours to travel up the carbs' vacuum line and back into the petcock (hence the fuel I found on the vacuum side of the petcock), and c) also seep out from the carbs overflow line and pool on the floor below the bike (fuel did pool under the bike for a few hours after the last ride - I'm guessing till the vacuum line equalized and shut the fuel drip).

    I'm a noob when it comes to motorcycle mechanics, but I am fairly good with fixing most other stuff, so a bit of reading on engine operation on the web and a dose of common sense made me come up with that theory.

    Can anyone poke a hole in my conclusion, or am I actually making sense?
    Leaking past the float needles is a very common issue with the Kat design. Your probably spot on for the issue.

    So clean the carbs, fix the leak issues. Do a bench test to verify the leaks are fully fixed before putting them back on.

    Then change the oil.

    Krey

    Leave a comment:


  • flyboy
    replied
    Originally posted by Kreylyn View Post
    I don't see the spacer that goes between the diaphragm membranes. Is it pulled off, a different color or something. Normally it's a white nomex spacer.
    It is indeed a different color - it's black. It's a hard one to notice on that photo so I'm highlighting it with red dashes here:


    I should probably add that I further investigated, made some interesting discoveries, came up with a theory, and am now wondering what you guys think of it:

    First of all, I found no holes in the membrane whatsoever. I reassembled the petcock, created negative pressure on the vacuum line, then sealed the vacuum end while depressurized to see if it would eventually equalize and arrest the fuel flow. It didn't - the valve appears to be working as expected. Yet I did find fuel on the vacuum side of the membrane, as evdent in the photo.

    So now I'm wondering if fuel never magically passed through the membrane, but instead condensated from the vapors that came up the vaccum line from the carbs.

    My theory is that perhaps carburetor is failing to achieve two things:

    1. Failure for vacuum to equlize immediately after the engine shuts off. That would keep the petcock valve open for a while after the shut-off, allowing fuel to flow to the carburetor.

    2. Failure of the carb to prevent fuel from flowing into the engine after it's shut off. That would allow fuel to a) seep around the rings into the crank case (hence the elevated fuel levels), b) cause gasoline vapours to travel up the carbs' vacuum line and back into the petcock (hence the fuel I found on the vacuum side of the petcock), and c) also seep out from the carbs overflow line and pool on the floor below the bike (fuel did pool under the bike for a few hours after the last ride - I'm guessing till the vacuum line equalized and shut the fuel drip).

    I'm a noob when it comes to motorcycle mechanics, but I am fairly good with fixing most other stuff, so a bit of reading on engine operation on the web and a dose of common sense made me come up with that theory.

    Can anyone poke a hole in my conclusion, or am I actually making sense?
    Last edited by flyboy; 07-17-2017, 05:25 AM.

    Leave a comment:


  • Kreylyn
    replied
    Originally posted by flyboy View Post
    It feels like I just replaced my petcock yesterday (in all reality, it was last August). So when the bike started having issues starting up, that was the last thing on my mind.

    Yet my petcock seems to be leaking copious amounts of fuel into the vacuum line (evidence so far - vacuum line smells like fuel, crankcase oil level went considerably up, gasoline dripping into a puddle under the bike, and lastly - petcock shows fuel on the vacuum side of the membrane - see pic below).

    What could be causing two petcock valve membranes to fail in less than a year? I burn 94 octane Chevron (with Techron), and have only added fuel stabilizer for winterization during this winter. Could it just be coincidence wrapped in bad luck?

    Puzzled.

    I don't see the spacer that goes between the diaphragm membranes. Is it pulled off, a different color or something. Normally it's a white nomex spacer.

    Krey

    Leave a comment:

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