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Cylinder pressure

Discussion in 'Tech Help' started by gorby, Jun 6, 2010.

  1. gorby
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    gorby Member

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    Hi,
    Recently got old bike and new pressure gauge. Tried to measure left cylinder - 110psi. Right - 60psi. Bugger.. measured left again - 60psi as well. Am I crazy? :mad:
     
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  2. Stout
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    What sort of gauge? Thread-in one, or one of the rubber cone ones you have to keep pushed against the plug hole?

    Were you testing it hot or cold, with other plug in / out and throttle wide open, closed or somehwere in between. Any variation in these things will make a big difference, as well any other inconsistency in how you conduct the test (how long you crank it for, etc)

    The state of charge of your battery will also make a huge difference. A battery that was cranking well to start with, but quickly dropped off could certainly explain your results.
     
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  3. Charlie
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    Charlie Member

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    I seem to remember there are a few other tricks for isolating problems when using a pressure tester. I think one of them is to add oil (through spark plug hole) and if your pressure increases then ring/cylinder/piston issues are likely. If pressure doesn't increase then it is possibly a valve problem, assuming it's a four stroke. I'm sure there would be a whole lot of information about it on the net. Have fun :)
     
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  4. gorby
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    gorby Member

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    I use rubber cone coz plug hole is deeeep and very narrow. will try today again with charged battery.
     
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  5. CT90
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    CT90 Member

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    Try to be consistent with it. The throttle should be wide open and crank both cylinders the same amount i.e. 4 turns of the crank.
     
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  6. Stout
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    It is well worth investing in a threaded compression tester. Even more so if access to the plug hole is difficult.

    I used a rubber plug one for years, so I think I'm qualiifed to say this: they're crap.

    Can't overstate the importance of a healthy battery though. Without this you're wasting your time. But then, there are a heap of other things too, like, you should check and adjust the valve clearances first, etc.

    The use of a little oil down the plug hole (between tests) as mentioned above is to try and isolate the cause of a low reading.The theory being that if the reading improves with the oil, the cause is likely worn rings (and the oil is helping them seal a little better). If the reading doesn't improve, it is likely something else, eg: valves, head gasket, cracked head, etc. You always hear about this anytime compression tests are mentioned, but I don't find it a particularly reliable (or useful) test and usually don't bother with it.
     
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