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  1. #1
    CB750 New Member g10rg10's Avatar
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    Aug 2018
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    New York City
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    1

    Rough idling low rpm + cam chain rattle

    Hello everyone. this is the first time I post but the site has been a great source for my problems so far.

    I have a 1979 CB750F that runs great. However, lately it has developed a new issue that I would like to figure out myself rather than being ripped off by some mechanic. I am not very mechabically inclined and have not really touched the engine with th exception of basic maintenance and tghteing the cam chain (once)

    So the current problem I have is roigh idling at low rpms. I need to raise the idle just above normal otherwise it makes horrible sounds and dies. However it runs just fine if you keep the rpm up.

    Sometimes it looks like it struggles to raise the rpms but once above 3000 it runs great.

    Combined with this I also have a noisy cam chain that just sounds worse than it should when the engine doesnt idle smoothly.

    I do not feel confident in my ability enought to pull the carbs and clean them myself. However I am also afraid of giving a couple hundreds of dollars to someone and it wont fix the problem.

    I know there are quite a few things going on here but I would really appreciate some expert advice on how to proceed and diagnose this before I drop serious money.

    Maybe you guys can convinve me to pull the carbs off and clean them myself...

    Anyway, nice to meet all of you.

  2. #2
    CB750 Enthusiast pidaster's Avatar
    Join Date
    May 2012
    Location
    MS
    Posts
    95
    Pull the carbs are rebuild them, sounds like the idle jets are clogged and the carbs may not be in sync. If you get it wrong the first time, just try again. I'm sure there's plenty of tutorials on web.

  3. #3
    CB750 Addict Dave's Avatar
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    Sep 2012
    Location
    Ca
    Posts
    497
    Carburation is the first thing everyone suspects and the last thing to ever go wrong.


    How old are the spark plugs? Replace them first because it's the cheapest thing to do.

    Still running rough? Check the valve clearances.

    On the DOHC, as the valves and seats wear the valve clearance gets smaller and smaller until it's gone and the valves can't close all the way anymore. The carbs will never work right if the valves are leaking because they depend on vacuum created in the cylinders to operate.

    This is the #1 thing that's kills DOHC CB750's, people never adjust the valves and eventually they leak and burn and the bike won't run right anymore.
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

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