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Thread: High rpm cutout

  1. #1
    CB750 New Member Silentyoda's Avatar
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    High rpm cutout

    So i have a 1981 DOHC CB750C, runs great at an idle; but as rpms climb while riding it cuts out as if it's not getting fuel/air. I suspect electrical to be the cause. Reasons being that I have completely disassembled all 4 carbs and rebuilt them. Charging system is all new and verified that it's charging. New regulator rectifier, rotor, and stator. Motor in the bike now I believe has 36k miles (I have a second original motor with 153k on it) To say the least, they bike has been rode an absolute ton. I suspect maybe the coils are the cause but I'm unsure of how to test for that. If you need more information please ask, would like to be able to get the bike up and running for this summer again.

  2. #2
    CB750 Guru amc49's Avatar
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    Set your valves first, nobody ever does it and the single biggest cause of them not revving fully out without missing. Number two reason will be pods if somebody has put them on and then after that ignition, they are weak there.

    Rebuilding carbs? Many do it 2-3 times to still have trouble with them.

    Set the valves to .005" instead of service manual .003", it allows like 3X time before having to do them again.

    Coils can be checked for absolute working but not for strength, replacing is all you can do to be sure there.

  3. #3
    CB750 New Member Silentyoda's Avatar
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    So I was talking again with my dad. It held the same problem from motor to motor so that can be eliminated as a fault in the system. valves on both are in good shape. What is the problem with pods? So long as the carbs are adjusted shouldn't be a problem, no? Where do we go for new coils? I'm thinking that is the culprit. After talking with my dad I remember that I started having this problem and switched the plugs to a cooler firing plug since I was doing a lot of highway, thought maybe the heat was causing firing issues, plugs are a cheap choice. switching to cooler plugs did seem to help for some time but the problem came back. Happens any gear roughly 5.5k rpm and above.

  4. #4
    CB750 Guru amc49's Avatar
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    You are talking death with the valve opinion you have there. Without a compression test you have zero idea of how the valves are, these burn them much easier than others as they do the opposite of normal, they close up to leak instead of getting looser and you set them every 4000 miles to stay on top of it. I'm sensing that not done here. Best way to burn them there is.

    That and pods if using them and the colder plug all conspire to make the CV slides not lift which then makes carb go rich to bang out at lower rpm. Very common and most do it on these. I've seen it a bunch.

    Buy coils if you want. I bought Accel back in the day for this issue and it didn't help at all. $150 for nothing. Ditch the colder plug, only makes it worse. The ignition IS weak but more important to get control of the carb slides or it never runs right and the rpm bang out limit will just get lower and lower. Then at some point the engine begins to sound slightly funny at cruise and burned valve time. How they all die.

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