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  1. #1
    CB750 New Member south yorks's Avatar
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    seems to be running weak

    i i have a 1981 cb750k with low mileage that has been stood for a lot of years i have cleaned the carbs out and replaced it seems to be running weak the plugs are nearly white do they run very lean or am i missing out on something could it be the fuel accelerator pump diaphram be anything or the air cut off valves
    the plugs are all the same colour thanks richard

  2. #2
    CB750 Guru amc49's Avatar
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    The plugs will run white for a while until they color up. If the valves have never been set you are likely burning them, a compression test is in order. What kind of mileage? They can burn valves under 5000.

    Euro models usually have no accelerator pump.

  3. #3
    CB750 New Member south yorks's Avatar
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    Quote Originally Posted by amc49 View Post
    The plugs will run white for a while until they color up. If the valves have never been set you are likely burning them, a compression test is in order. What kind of mileage? They can burn valves under 5000.

    Euro models usually have no accelerator pump.
    hi i have only done 60mls since cleaning carbs etc the bike has only done 3000mls it is a usa model i will check the valves i thought with the low milage they would be fine i checked the compression they are all above 150psi cold it may run leaner than the old sohc models i do not know as this is my first dohc i have owned l will take it on a longer ride. richard

  4. #4
    CB750 Guru amc49's Avatar
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    I have personally burned valves on a new one at 3000 miles. Don't set them as close as the service manual says, use .004"-.006" and shooting for .005" and they last MUCH longer. The service manual allows for .002"-.004" and .002" turns out to be zero when the engine is running and they burn. You throw away a solid .002" of whatever measurement you get as it is not real.

    Brand new bikes were supposed to recheck/set valves at 600 miles after run-in but most of them never got it, then every 4000 after that. Setting at the service manual numbers may not make 4000, the going to .005" definitely does and can go to as long as 24000 before problems, I have done it.

    The cam caps are looser than the valve clearances and it allows the fairly strong valve springs to push the cams around in the caps while hand turning the motor to make your measured number not be what is real at engine running. Where the .002" went.

    OEM compression number is 170 psi.

    Remove or defeat the idle limiter caps and adjust the mixture screws to 2- 2 1/2 turns and the bike will idle better and pull away a bit better.

  5. #5
    CB750 New Member south yorks's Avatar
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    hi checked the valves the exhaust valves are all between 005"/006" inlet 003"/004" checked the compression again all around 160/170psi
    will give it a 100 mile run in the next few days i have noticed that it revs a lot higher at 50mph than the sohc models is it worth putting a smaller rear sprocket on
    richard

  6. #6
    CB750 Guru amc49's Avatar
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    It's up to you............

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