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  1. #1
    CB750 Enthusiast John Luke's Avatar
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    Still need help with tuning CB750C carbs

    I guess my question is which jet controls WOT?


    I have totally rebuilt my 1981 CB750C. I have done the timing and the valve clearances. I have 4into1 delkevic exhaust and individual K&N pod filters. The current jetting I have is a 135 secondary main and a 80 primary main. With 3 turns out on the a/f mixture screw. She idles great and roars to 12000 + rpms. But when I give her full throttle on the highway she seems to be bogging. She’ll get to like 90mph and won’t go any faster. Im tempted to put tape on the filters to see if that helps. Hopefully that is a good explanation.

    Any ideas or help would be appreciated.


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  2. #2
    CB750 Guru amc49's Avatar
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    You are in pod hell and what they do. Why we tell you not to do them.

  3. #3
    CB750 Enthusiast John Luke's Avatar
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    I'm ordering some air boots. Gonna throw her back together with the airbox on and see if that solves my problem.


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  4. #4
    CB750 Enthusiast John Luke's Avatar
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    Still a bit frustrating because it seems you said that you've done multiple bikes with like say a 75/115 set up and had absolutely no problem.


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  5. #5
    CB750 Guru amc49's Avatar
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    Not making that up (did it on 4 myself), the bikes run great other than the last bit of pullout is not there in high gear ONLY, who needs the 125 mph there, 110 is fine. Others with a solid motor have gotten there too after I told them the setup but the engine MUST be in prime shape, most aren't.

    I'd bet the valves are leaking a bit too, I have never pulled one apart that wasn't if it had 20K miles or more. The exhausts go through hell and the rev range then dies up high. No way you will have a 40 year old bike run fine up there and the pods make that worse and why everybody goes crazy. The only way to get away from that is valve job and changing the exhausts, they cannot be ground, only lapped and doing that too much makes it worse too.

    The engines were made in a era of low lead fuel but even low lead gave massive advantages to valve life as compared to no lead now and ethanol only makes it worse. The valves are only thin coated with nitride and it goes away with lapping or fairly low wear and then the super soft steel underneath then burns like lightning. Add to that needing the OEM airbox way more than most engines due to the type of CV they use (labyrinth instead of rubber diaphragm, it leaks more) and the carbs with aircuts and double sided idle circuits that clog to defy even the best carb rebuilding guys and you see why the engine type is so expensive to get back to running right as compared to most other fours. Why the parts network never developed like for the SOHC.

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