Compression vis-a-vis adjusting the carburetor
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Thread: Compression vis-a-vis adjusting the carburetor

  1. #1
    Junior Member Dwmdsen's Avatar
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    May 2019
    Pleasant Hill, CA

    Compression vis-a-vis adjusting the carburetor

    I'm resurrecting a '82 XT200 and an finding getting the bike to idle is a challenge. Compression is only 90 psi and I'm looking for validation that this could be some of my issue. I added a tablespoon of motor oil into the cylinder and compression picked up to 130 psi, so it would appear rings are suspect.

    Current setup: stock intake/exhaust, valve adjustment is within spec, carburetor has been fitted with standard OEM jets.

  2. #2
    Senior Member assquatch20's Avatar
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    Jul 2010
    You could be right, but I'd make very sure the carburetor isn't the culprit. Playing with jetting or just cleaning them again may help. Did you do the compression test with the engine cold or when warm? Compression can go up for a running engine. Doing a leak down test might be more telling. If you're willing and able, measuring the bore is probably the best way to be sure. I don't know if it's the same as the TW's numbers, but there's an acceptable range of wear in the cylinder and then there's what's out of tolerance. A service manual and a bore gauge can solve this if you're willing to go that far, but I'd try to check what's easily accessible first.

  3. #3
    Senior Member Fred's Avatar
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    Dec 2011
    Turtle Wrangling the Sierras
    Try turning up the idle until bike can sustain itself, then ride it around and note how the bike is running overall.
    However it sounds like the '82 would really benefit from a top end re-build...appropriate new rings in a honed cylinder, new base gasket, maybe lap valves with new seals, new if not stiffer valve springs as long as head is off, etc.
    assquatch20 likes this.
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  5. #4
    Senior Member Sthrnromr's Avatar
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    Oct 2018
    Rural, South Carolina
    If I saw compression go from 90 to 130 without variable of engine temp, I’d lean towards rings and top end rebuild for sure. Especially on a new to you 82, unless you have the history and know the last time it was done (if ever). But, as others stated, carb tinkering is free and a place to start. My 2 cents.
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  6. #5
    Senior Member elime's Avatar
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    Jul 2010
    San Jose, CA
    You say resurrecting.......I am guessing one or both of the compression ring is stuck. Getting the motor running for a while might free it up. I would try some Chemtool B12 in the spark plug hole and letting it sit overnight before I tore it all apart.
    Long live the internal combustion engine!

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