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  1. #1

    Join Date
    Jun 2019
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    2

    Default New to bikes, CB750 advice needed (low compression)

    Hi all,

    I'm working on a 1982 CB750C, It ran and idled great but would misfire under heavy load.

    So I just ran a compression test and here are the numbers.

    Cyl 1: 95
    Cyl 2:120
    Cyl 3:110
    Cyl 4:120

    Cyl 1 (when oil added): 110

    What do you guys think? The spec lists 170psi +-14 psi. Seems worrying, thanks for any advice you can give.

  2. #2
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    Have you adjusted the valves???? That is critical on those motors.......

  3. #3
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    What is the history with the engine? Raced hard for the past 37 years? Sitting for a while? Never been apart?

    Did you have the throttle wide open during the compression test? (guy last week had low compression and guess what, re-test with WOT and there it was!)

    Got a manual?
    DOHC CB750 Honda Factory Manuals
    http://www.dotheton.com/forum/index.php?topic=44399.0

    What about a parts 'book'?
    https://parts.southernhonda.com/Hond...0C/Engine.html

    This may be helpful:

    Valve adjustment/check dohc
    Q: "Does anyone have step by step instructions for checking the valve clearance on a dohc cb750?"

    A: "Download the FSM from the front page of the site. You should really get familiar with it. At the very least the inspection/adjustment chapter and the addendum to that section for your year. ...here is a step by step video I made on how to do it."
    http://vintage-and-classic-hondas.45...td4042213.html

    A leakdown tester can also be helpful with diagnosis; build your own easily here;

    Building and Using a Cylinder Leakdown Tester
    Click image for larger version. 

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    https://www.motorcycleproject.com/text/Leakdown.html
    Last edited by TriNortchopz; 06-28-2019 at 1:37 AM. Reason: WOT?

  4. #4

    Join Date
    Jun 2019
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    Thanks for your replys

    1. No history on the engine sadly, I bought it from my neighbor, who had no history on it either. It was sitting for awhile until I got it and I rode it for probably 50 miles or so.
    Unknown mileage but over 30k.

    2. The compression test was ran with the carbs off so throttle was not an issue.

    3. I have a paper copy of the manual that has been very helpful. But that parts book seems useful as well.

    4. It was my understanding that the valve clearance would not effect compression because if anything the clearance is too large causing the valve not to open all the way but should seal fine. Is this correct?

  5. #5
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    Quote Originally Posted by Smac11 View Post

    4. It was my understanding that the valve clearance would not effect compression because if anything the clearance is too large causing the valve not to open all the way but should seal fine. Is this correct?
    Nope that is incorrect.....

  6. #6
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    Quote Originally Posted by Smac11 View Post
    4. It was my understanding that the valve clearance would not effect compression because if anything the clearance is too large causing the valve not to open all the way but should seal fine. Is this correct?
    With the DOHC engine, the valves hammering themselves into the valve seats and the cups and shims hammering themselves together is what changes the clearance, and it tends to tighten rather than loosen. Also, sometimes if the engine gets wound really tight, a valve can hang open just long enough to be kissed by the piston, bending it ever so slightly (or more.) The best way to tell without disassembling the engine is with a leakdown test.

  7. #7
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    Quote Originally Posted by MOTher View Post
    With the DOHC engine, the valves hammering themselves into the valve seats and the cups and shims hammering themselves together is what changes the clearance, and it tends to tighten rather than loosen.

    Yes sir, He is exactly right.......

    Adjusting the valves in a DOHC bike is critical......

  8. #8
    sunnysunny
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    Really your post is really very good and I appreciate it. It’s hard to sort the good from the bad sometimes.You definitely put a new spin on a topic thats been written about for years.
    wuxiaworld

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