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  1. #1
    CB750 Member Jahred's Avatar
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    Valve Lash ... how important?

    Working on non-running 82 CB750SC/Nighthawk.

    Replaced Coils and now firing on 4 instead of 1 cylinder. Still kills with throttle. Guessing Pilots.

    In meantime I wanted to check valve lash as I see many suggesting this is critical.

    spec is .08mm +.05 so 8 to 13, both intake and exhaust. All my valves are wider than .08.

    I'm averaging .1775 I intake with gap ranging from .14 to .23.

    On exhaust side, 4 within spec; averaging 13.625 and ranging .1 to .18.

    I thought these could be adjusted like my Cobra with Lock Nuts. Huh. I'm reticent with my modest skills to actually pull the camshafts and do this shim thing.

    Seems to me looking at feeler gauges that even .13 vs the .23 does not change the duration for jack and not likely my problem if engine won't accept any throttle. Idles fine. Higher RPM with choke.

    Are these Valve Lash differences worth worrying about?

    What's a reasonable price if I haul it to a mechanic for tune-up/carb cleaning?

    Thanks in advance.

  2. #2
    CB750 Guru amc49's Avatar
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    Better spec range is .10-.15 mm., or .004"-.006". Forget the service manual. You are too close to burning valve at .08. Cams do NOT come out to change shims. If they are that loose then somebody else may have done it sloppily and found the valves are already burned. The engines tend to CLOSE UP the valve clearance rather than get looser and why I suspect. Running a compression check is of worth as many are way down and why they don't run right. looking for 170 psi. You need to close the loose valves up.

    Carbs are often a problem too and stuck closed pilots are common. As is clogging all over the carbs and the aircuts often are ruptured too. Many people including the techs have to clean the carbs more than once to get them fully open. Some never get then open.

    Your issue will be more one of who you can find to do the work if they are already familiar with early Honda DOHCs, they are avoided by most people to make money off of.

  3. #3
    CB750 Member Jahred's Avatar
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    Thanks Guru!

  4. #4
    CB750 Member Jahred's Avatar
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    AMC, if I could impose … started to pull carbs … Haven't got that all figured out … but I noticed after pulling boot on #4 that butterfly stays shut … they all do, and do not respond to throttle. They partially open with choke … choke linkage looks a bit iffy and opens or closes about 75% … but throttle does nothing to butterflies … should they not open with throttle even when bike is not running? Recall bike idles but stalls with any throttle. Carb Newby.

  5. #5
    CB750 Guru amc49's Avatar
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    If you are talking about boot at BACK of carb that is not a butterfly, it's the slide which only lifts with vacuum and engine running. stick a finger in and under it, it should lift easily but go back down at finger removal. CV carbs.

    The true butterfly is up front and you won't see it until carb bank is entirely removed. Work throttle and you will see it working linkage in the middle of the carbs, that is the front butterfly shafts working.

    Carb works by opening front butterflies but the back slides do not begin to open until the engine has demanded them to. They are automatically flow controlled.

    Keep all carb parts SEPARATE and marked and most certainly the slides and top caps they go with! Critical move there, they are matched sets.

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  7. #7
    CB750 Member Jahred's Avatar
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    AMC, another quick fix hope dashed! Thanks a bunch for your help!

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