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cylinder compressions. when is it time to overhaul??

Discussion in 'Yamaha 250cc In-Line 4's' started by blueyedjackel, Aug 19, 2013.

  1. blueyedjackel

    blueyedjackel New Member

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    hey gremlins. . . . whats the correct procedure for cyclinder compression testing. is there any way to get false readings or the best readings etc.
    just did cylinder compression test on my 1992 3ln fzr. manual says 142psi (981kpa/400rpm)was normal. mine averaged round 125psi. (11.3% down) with 13,000km on the clock. is this typical and at what psi would you start opening up the engine to renew things?
    what have other people been getting?
    also what should the psi reading range be between cylinders?
     
    blueyedjackel, Aug 19, 2013
    #1
  2. dave

    dave Well-Known Member

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    From what I gather with other users is that around 100psi is not uncommon. Below that the problems start.
     
    dave, Aug 19, 2013
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  3. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Difficult to know how accurate your compression gauge is unless it comes with a certificate of accuracy. So comparisons should take that into account.
    I have read on another forum where a member tried a variety of compression gauges, all supposed to be good quality, and all gave different result.
     
    maelstrom, Aug 20, 2013
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  4. blueyedjackel

    blueyedjackel New Member

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    guage is brand new never been dropped or over pressured. its an actron brand whatever that means.
    well i did the first test just cranking the motor over with all the leads disconnected so the bike wouldn't fire up and got round 120-125psi average. then i put the other leads back on and cranked it again and the bike fired into life (comp tester still in No 1 spark plug hole) and got similar reading. no surprises there.
    while idling i did 5 tiny blips on the throttle(approx 4500rpm) and each time i blipped the throttle it pumped the needle up and up ever so slightly till it stopped at a new record 134psi.
    please keep in mind the pressure rise wasn't just rpm related, for example when i slowly revved to 5500rpm it wouldn't keep pumping the needle up. it was more the sudden twist of the throttle causing a sudden acceleration of the engine that had that effect. (hmmm <!-- s:dot: --><img src="{SMILIES_PATH}/th_dot_dot_dot.gif" alt=":dot:" title="dot dot dot" /><!-- s:dot: --> )
    the manual did state the figure they had was at 400rpm which i assume is the speed the engine turns over at when cranking the starter motor.

    what ya rekon boffin heads <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->
     
    blueyedjackel, Aug 20, 2013
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  5. dave

    dave Well-Known Member

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    Compression test is usually done on the starter motor crank over. That is where it will show up poor rings and valves the most. You get a consistent comparison speed and everything has time to leak if faulty.
     
    dave, Aug 20, 2013
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  6. Gosling1

    Gosling1 New Member

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    as long as all the readings are within 5-10% of each other and still over 100psi, then you don't really have any issues.

    The point of compression testing is to make sure that all the pistons/rings etc are wearing at the same rate. If you have say 3 cylinders at 120psi and 1 cylinder coming in at say 80 or 90psi - then clearly there is a problem with *that* specific cylinder. It could be poor valve sealing/clearance issues, or maybe a piston on the way out - that sort of thing.

    <!-- s:cool: --><img src="{SMILIES_PATH}/icon_cool.gif" alt=":cool:" title="Cool" /><!-- s:cool: -->
     
    Gosling1, Aug 21, 2013
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  7. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Compression test must be done with the throttle fully open and a warm engine.
    Disable the ignition.
     
    maelstrom, Aug 21, 2013
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  8. dontz125

    dontz125 Active Member

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    With CV carbs, 'throttle fully open' means you have to pull the airbox off and physically lift the slides with your finger. Just twisting the grip won't accomplish anything, and you'll get an erratic and artificially low reading.
     
    dontz125, Aug 22, 2013
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  9. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    <!-- s:| --><img src="{SMILIES_PATH}/icon_yeahthat.gif" alt=":|" title="Yeah That" /><!-- s:| --> to what Dave, Gosling1 and DonTZ125 said, and almost me too <!-- s:oops: --><img src="{SMILIES_PATH}/icon_redface.gif" alt=":oops:" title="Embarassed" /><!-- s:oops: -->
     
    maelstrom, Aug 22, 2013
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  10. blueyedjackel

    blueyedjackel New Member

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    so do i take my air box off, stick something down the carb throats to hold slide open and do the test again?

    funny ive never heard of anyone doing it that way before?
     
    blueyedjackel, Aug 22, 2013
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  11. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Given that your result was not bad with the slides closed, and that you can expect it to increase a little when you open the slides, it all sounds pretty healthy. Refer to Gosling1's post about the pressures being different and your good to go. IMHO
     
    maelstrom, Aug 22, 2013
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  12. blueyedjackel

    blueyedjackel New Member

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    ahhh so maybe that's why i was getting higher readings when i had the compression tester connected and the bike while running and i was blipping the throttle which would have been opening the slides which is the same as the set up you guys said to do. makes sense to me <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->interesting
     
    blueyedjackel, Aug 22, 2013
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