80 CX 500: COMPRESSION???Hey guys. My CX 500 is only putting out 70psi / head right
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  1. #1
    Junior Member
    Join Date
    May 2014

    80 CX 500: COMPRESSION???Hey guys. My CX 500 is only putting out 70psi / head right

    Hey guys.

    My CX 500 is only putting out 70psi / head right now. The bike feels pretty slow out of starts and has no real top end. Makes plenty of noise but not get up and go. After extensively tuning and cleaning carbs and ruling out other options I figured this was it. (Checked compression last...duhhh)

    Question: Is 70psi / head low enough to drastically feel a loss in power? I know its not ideal...

    And the manual calls for 171 psi....which seems ridiculous.

    I have rebuilt engines before with brand new top end components and usually get 100-110 when the manual calls for 160, as an example. So im kind of skeptical...


  2. #2
    Super Moderator CXPHREAK's Avatar
    Join Date
    Aug 2010
    South Australia
    Mine runs 170 hot WOT on a compression test.

    These motors will run fairly happily down to about 100 or so PSI but below that they're in trouble.

    Was your motor warm and the throttle wide open on your test?

    What was the other side?

    Also, with new rings you won't see full compression for a thousand miles or so.
    Rebuilding Handlebar Switches

    Rider 81 500C, Thingy Born a 78Z. Ozdeluxe born an 80 shadow.

    GET A MULTIMETER RTFM I am a CX500. If you maintain me I will run forever.


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  3. #3
    Senior Member
    Join Date
    Mar 2013
    Wellcome to this forum, Pat

    Make a new compression test.

    1. "Normal" test (with full throttle)

    2. Spray some oil into the spark plug hole and check it again.

    If the compression increases with the second test, it'll show You that the piston rings are worn. (Bad sign, lot of work)
    If the compression does not increase, it's a sign that the valves do not close. ("Good" sign, easy to repair.)

    Mine runs 170 hot WOT on a compression test.
    Dito ;-))
    Last edited by guelli02; 05-11-2014 at 03:51 AM.

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  5. #4
    Junior Member
    Join Date
    May 2014
    - Pulled both plugs
    - Insert compression tester in head (other head open to atmosphere)
    - Crank engine over w/ starter for about 15 seconds
    - 70-75 PSI in L head
    - 75-80 PSI in R head

    Engine was COLD

    Checked valve clearances, all good. But maybe the valves themselves are bad?

    42,000 miles on it too :-\

  6. #5
    Senior Member
    Join Date
    Apr 2011
    Lexington NC
    borrow another gauge to compare it would suck to condemn a motor due to a bad gauge

    or better yet get a leak down tester

    did you hold the throttle wide open?

    if so the put some oil down the cylinder and do it again

    Order The Quick Build Exhaust Pipes


    Way to many bikes to maintain this list LOL !!

    email me at murray at murrayscarbs.com

  7. #6
    Senior Member Spruce Laker's Avatar
    Join Date
    Feb 2014
    Spruce Lake sk Canada
    If you get a good indication from all the advice above the next thing is late spark due to low capacitance in your cdi. That will make your bike run hot backfire and limit your revs.

    1980 cx500 Deluxe

  8. #7
    Senior Member Blue Fox's Avatar
    Join Date
    Aug 2010
    Columbia, MO
    Make sure that you are doing the compression check correctly. Here is what our Wiki says: Link to compression test
    1983 GL650I, 1980 CX500, 1974 CB350F, 1973 CT90

    Need information? Try our new Wiki site: http://cx500forum.com/mediawiki/index.php?title=Main_Page

  9. #8
    Senior Member Manzanita's Avatar
    Join Date
    May 2011
    Woodland, California
    My new Harbor Freight gauge measured 80 psi on both cylinders on my cx500, which runs great. It's the gauge, I assume, because my ascot read 90 psi on the good cylinder and 45 psi on the bad cylinder (head gasket blown).

    So yeah, be sure your gauge is good. Try it on a bike (or car) that runs well if you don't have another gauge.
    1981 CX500 Custom
    2014 Zero S

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