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  1. #1
    CB750 New Member RoderickYammins's Avatar
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    1979 CB750K Engine Misfire Quest

    Greetings,

    I've just purchased a CB750 with 16k miles. Previous owner chopped it up quite a bit, but I'm going to try to get her running right.

    So, What's Going On?
    Well, when I bought this bike, it sounded fine, all headers were blazing hot, and we hauled it back. The gave me a baffler to put on, which I did, and next time I started it I took it out for a ride and it was excellent, very powerful and smooth. I come back and now I notice cylinder 2's header is not quite hot, and the engine is making a knocking sort of sound.
    I take the nasty carbs apart and find it's got two different sized main jets. Two carbs have 68, the other 2 have 74. All four have 112 secondary lmao.
    Currently awaiting rebuild kit and new jets. Spark plugs were all black as well.
    My question is what could have caused such a quick change? And, if the rebuild does not provide respite, would the next best thing be to check for compression?
    I have a video of the sound. I can upload it if it would help.

  2. #2
    CB750 Guru amc49's Avatar
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    Run compression to make sure valves not burned, they burn very quick if not kept set and were supposed to set at 600 miles, most never got it. Valves need to set at .005", they burn at .002" as it is actually zero. Bike sitting clogs up idle feeds to run rich, I know it's backward but what they do. If ethanol in your fuel it's even worse.

    Mine burned valves in less than 4K miles when new, then it makes owners think the carbs are messed up but then they may be if bike sat. If the original airbox is gone that's even worse.

  3. #3
    CB750 New Member RoderickYammins's Avatar
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    Will do.
    It unfortunately has pods, which I know does not play well with the CV system. I will find a replacement air box eventually and bring the engine config back to stock.
    At the moment I am awaiting a carb rebuild kit from vintagecb750.
    I will update the thread once Iíve done that and compression test if carb does not provide fix.
    Thanks for the advice!

  4. #4
    CB750 New Member RoderickYammins's Avatar
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    **Update**
    So I ran a compression test on a bone cold engine.
    Got 90 psi accross the board. Cheap tester from Harbor freight, so actual reading may be different. However, it's the precision that counts.

    Still awaiting carb rebuild kit and jets.

  5. #5
    CB750 Guru amc49's Avatar
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    A typical result and what gets people in trouble. Anything below 100 is a dead cylinder and it would indeed be odd to have all 4 the same number, well nigh impossible. Points at a crap gauge. I always run 3-4 tests on each cylinder and throttles must be held wide open at doing it. The book new engine number is 170 psi and most decent engines will read over 130. Too much 'precision' screams a faulty gauge, it is extremely odd to get the same number on all of them. All 4 plugs removed and I also use a car battery to spin the engine, bike battery will begin to run down with that much starter use.

  6. #6
    CB750 New Member RoderickYammins's Avatar
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    Carbs have been rebuilt and I purchased a stock airbox off ebay.
    Ran like an absolute champ and all headers were hotter than the sun.

  7. #7
    CB750 Guru amc49's Avatar
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    Ten million degrees? The exhaust pipe metal will be liquid at about 5000.

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