Compression help
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Thread: Compression help

  1. #1
    Junior Member Foxie117's Avatar
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    Compression help

    My 125 won't start it's been running ruff for ages problems starting and a weak to intermittent spark. I've got new coil, new plug ( gapped correctly ) I've cleaned the contacts on the coil wires. But still no start. I'm trying to compression test it but the figures I've got are way off. Image 1, from a tw Manuel. And picture 2 my test with open throttle. Is the figures in n°1 right? Any ideas why mine are so low?

    Thanks in advance
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  2. #2
    Super Moderator Purple's Avatar
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    Try a little engine oil down the plug hole and repeat the test – if the results improve significantly, your rings are worn

    It could also be the valves – either one of them is too tight, or not sealing properly, or even bent
    Miaugi and Fred like this.
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  3. #3
    Senior Member elime's Avatar
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    Make sure your valve clearance is correct.

    Sometimes when you remove the spark plug it knocks a little carbon off and it hold the valve open a little bit. If you reinstall the plug and can get it running that is enough to solve that problem.

    Pour a little gas in the spark plug hole and try to start the engine. If you really want to go all the way, put some gas in the spark plug hole, install the spark plug, heat the head around the spark plug with a propane torch until it is barely warm (no where near hot), then try to start it. Make sure the key is on. Good luck.
    socalnative likes this.
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  5. #4
    Senior Member socalnative's Avatar
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    You probably have more than one issue unless it's just that the valve clearances are too tight.
    Start with the basics. If you can check or adjust the valves do that. The weak spark isn't going to matter with compression that low.
    When you say it's been running rough for ages, what do you mean? Hard starting, lack of power, missing, doesn't rev up, won't run without the choke on?
    Please elaborate some. It might help diagnose your issues.
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  6. #5
    Junior Member Foxie117's Avatar
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    Quote Originally Posted by socalnative View Post
    You probably have more than one issue unless it's just that the valve clearances are too tight.
    Start with the basics. If you can check or adjust the valves do that. The weak spark isn't going to matter with compression that low.
    When you say it's been running rough for ages, what do you mean? Hard starting, lack of power, missing, doesn't rev up, won't run without the choke on?
    Please elaborate some. It might help diagnose your issues.
    Hi. Its most of them. Trouble starting, power loss, bogging/ missing. I've thought it might be the CDI module, but at £150 for one even for 2nd hand, I can't afford that just to see if that will fix it.

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