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Thread: Cylinder stops firing when bike heats up

  1. #1
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    Default Cylinder stops firing when bike heats up

    Hi -

    GS450


    Bike mostly sat during winter. No issues last season. Now after a few minutes of warming up and idling fine, the right cylinder starts sputtering and eventually stops firing after a couple minutes. Will only start firing again when the bike is pushed to 2-3k rpm, but not on low rpm or idle. After letting the bike sit and cool down the cylinder will fire again for a few minutes until heating up.


    What I have done -


    1. switched spark plugs / confirmed there is spark in that cylinder
    2. switched out fuel and added fuel filter
    3. cleaned out airbox / intake as much as possible
    4. cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)
    5. cleaned out fuel hoses as much as possible


    I did check the compression and it was 150psi on the good cylinder and 120 on the bad one, so I'm wondering if that could cause the issue or if that is a separate problem altogether?


    What could cause it to work while cold but then the cylinder start failing after a couple minutes? Is it pulling in dirty fuel and I just need to clean the fuel passages better? Does the carb need to be retuned? Or does this just sound like a compression issue to you?


    Any tips appreciated.


    Thanks

  2. #2
    Nessism's Avatar
    Nessism is offline Forum LongTimer GSResource Superstar
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    Default

    Is the tank free of rust and debris?

    Have you checked valve adjustment?

  3. #3
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    Quote Originally Posted by epytus View Post
    I did check the compression and it was 150psi on the good cylinder and 120 on the bad one, so I'm wondering if that could cause the issue or if that is a separate problem altogether?
    That doesn't sound like a problem to me.
    1982 GS1100E V&H "SS" exhaust, APE pods, 1150 oil cooler, 140 speedo, 99.3 rear wheel HP, black engine, '83 red

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  4. #4
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    Your jets are constantly clogged?
    That's your issue.
    You need to clean the inside of the gas tank and then seal it
    Caswell's, or POR 15

    Oh, and check your valve clearances to determine proper compression
    After all that, it could be the ignitor if your bike is 80 or newer
    1978 GS 1000 (since new)
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  5. #5
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    I mean yeah, this...

    4. cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)

  6. #6
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    “cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)”

    this makes no sense.after only few minutes of run time . Is your petcock vacuum hose connected to this right cylinder?
    1981 gs650L

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  7. #7
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    What is the material clogging the jets? Rust? Tank liner? Plastic? Metal bits? Gasoline varnish (probably not)?
    '83 GS650G
    '83 GS550es (didn't like the colours in the 80's, but they've grown on me)

  8. #8
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    4. cleaned out carb jets (every time I take off the carb the jets are clogged, even after couple mins of running)
    ...that right there is your culprit, you'll have to "debug" your fuel system from the carbs right up into the tank. I had similar problems; in my case the carbs wouldn't stop overflowing.
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  9. #9
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    Quote Originally Posted by Nessism View Post
    Is the tank free of rust and debris?

    Have you checked valve adjustment?
    Yes on both. Just found the time to do a valve adjustment this weekend. Same bad results after firing up the bike.

    What is the material clogging the jets? Rust? Tank liner? Plastic? Metal bits? Gasoline varnish (probably not)?


    I shouldn't have said the jets were 'clogged' when I check them. One jet isn't fully clear but it appears to have some leftover fuel that is sitting in the jet every time I pull it out. Meaning instead of being able to fully see through the jet, it is slightly narrower diameter but you can still see through it. No obvious signs of rust or any foreign substances - just fuel. Not sure if that's to be expected?

    ...that right there is your culprit, you'll have to "debug" your fuel system from the carbs right up into the tank. I had similar problems; in my case the carbs wouldn't stop overflowing.
    Wouldn't the dirt still be there hot or cold? Unless the mere act of cooling down releases the clog. Even if so - would it really do it like clockwork every single time the bike cools down? I also installed a fuel filter between the tank and the carbs so the dirt would have to be in the fuel lines if that is the case. Airbox is already cleaned out.

    If it IS dirt and a clogged carb - what about the bike heating up makes the carb dirty? It can't just be pulling in new dirty fuel, because if you just let it sit and cool down it will run on that same fuel.

    If it's NOT dirt (or other foreign substance) - does this look more like an internal engine problem?

    Thanks
    Last edited by epytus; 04-25-2021 at 07:51 PM.

  10. #10
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    Swap the coils side to side and see if the right cylinder works and if the left cylinder begins misfiring.
    1980 GS750E (bought used June,1983)
    1968 CB350 Super Sport (bought new Oct,1968)
    1962 CA77 305 Dream (bought used Feb,1963)

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