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XJ750R Seca low compression

Discussion in 'XJ Technical Chat' started by Bauerxj750, Jan 17, 2016.

  1. Bauerxj750

    Bauerxj750 New Member

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    I have a 1982 Yamaha XJ750R Seca with 80 pounds of compression across all cylindes within 5 psi....freshly rebuilt motor, new rings, lapped valves, valve clearances adjusted, all new gaskets. Before the rebuild i was getting the same amount of compression thats why i took it apart to rebuild it....now its the same, warmed up the motor to operating temp and checked all cylinders with throttle wide open....why is it so low?
     
  2. jayrodoh

    jayrodoh YimYam

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    Did you verify your gauge is good?
     
  3. XJ550H

    XJ550H Well-Known Member Premium Member

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    you need to wait for your rings to seal into the piston walls, this is common with new rings and expected.
    compression test should be done on a cold motor. test then add oil to cylinder test again.
    yes it could be your tester
    read the information over load link for proper method and specs
    The Information Overload Hour
     
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  4. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    +1 ^

    Test the gauge by checking compression on another engine, or borrow a gauge from your local auto parts store.
     
  5. Bauerxj750

    Bauerxj750 New Member

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    I was going to borrow a friends gauge or an auto parts stores to see if the reading changes at all and the repair manual i have says to check compression at full operating temp...which should i do? Cold or hot? Would that make a big difference? Also how long should i wait for rings to seal into the walls before i try testing it again? Thanks for the help.
     
  6. rocs82650

    rocs82650 Well-Known Member

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    My apologies if you feel this question is insulting because it's not intended that way. Are you certain you're doing a proper compression test? Follow the link in 550's post to do so.

    Gary H.
     
  7. jayrodoh

    jayrodoh YimYam

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    Opinions will vary but my experience (non XJ) is at least 500 miles. Engine should be warm when checking compression if possible. When valves were lapped, was the head tested for leaks? Check clearances again, freshly lapped valves will "settle" quicker than during normal operation.
     
  8. Bauerxj750

    Bauerxj750 New Member

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    The head was not checked for leaks, i will check to make sure gauge is correct then after riding it for a while i will check compression again, if the problem still persists i will re check the valve clearances, i will also be sure to follow the instructions provided in the link by 550 when doing my tests. Thanks for the help, ill get back to you with the results.
     
  9. XJ550H

    XJ550H Well-Known Member Premium Member

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    from info overload steps omitted for clarity
    - COMPRESSION TESTING:
    a) make sure the engine is cool/cold.

    b) remove all spark plugs, and then stick the plugs back into their caps and make sure the plugs are grounded to the cylinder head (or even better, disconnect your TCI unit).

    c) remove the airbox filter lid and the air filter.

    g) Let the starter cool down for a minute or so, then do the next cylinder, etc.

    h) If the readings are below spec, then shoot about a teaspoon amount of motor oil into each cylinder, crank the engine over a few revolutions with the starter (to spread the oil around), and then re-test each cylinder using the above procedure.

    i) now re-install the spark plugs, let the engine warm up to operating temp.

    k) compare the two results and analyze.

    from fsm
    upload_2016-1-18_0-26-40.png

    so how you do it is up to you
     
  10. SQLGuy

    SQLGuy Well-Known Member

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    Does the information overload provide an expected "good" range for a cold engine? If not, I would go by the service manual instead.

    Also (in general, not the issue here) note that compression numbers will be reduced at higher elevations. The shop manual numbers are for sea level.
     
  11. XJ550H

    XJ550H Well-Known Member Premium Member

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    no but it does have the formula for different altitudes.
     

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