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  1. #1
    CB750 Member John Luke's Avatar
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    4-1 delkevic exhaust jetting

    I am working on a 1981 CB750C. I have a 4-1 delkevic exhaust on and the original air box. Can anyone tell me what jets and air/fuel mixture settings yall use? It seems to run okay. But when I give it throttle the rpms lag a bit b4 going down and there is a little bit of backfiring. Any help would be great. I have stock jets in rn.

    Thanks!

    Stock main jet is #102

  2. #2
    CB750 Guru amc49's Avatar
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    Check PM.

  3. #3
    CB750 Member mitchi's Avatar
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    Mind posting, I would like to know the answer too.

    I have the same exhaust, just haven't installed it yet.
    "If it ain't raining, I'm riding"

  4. #4
    CB750 Guru amc49's Avatar
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    I basically told him the same thing I'll tell you, and that is that there is no way to be accurate on jetting guesses as the carbs are CVs and depend way too much on overall engine condition to open up the slides as they are supposed to. When the engine vacuum drops off due to nobody ever setting the valves (highly likely and usually the first thing I ask after a compression reading that shows the bad effects) the carbs tend to go richer due to slides hanging low because they do not have enough vacuum to open fully. On CV carbed bikes you absolutely do not have a throttle that is positively tied to what you hand does with the grip, they can be two wildy different things to really throw off all jetting guesses. Why so many people who simply think you must have more jet mess the running up, the first thing it does is kill the upper end revability. The engine won't pull out past 9500 like it should. For something as light as a header only and that with no mods (like keeping the full packing in place to kill the header draw) to make it really 'work' it's enough to let the CVs simply self adjust and what they do to a point with lightly modded engines.

    That pipe is not optimum anyway as it does nothing with the can after the header tubes join, the length is close to wasted there and what the companies do now to make them quieter. The older school header with a true megaphone end makes more power as the can is then used to increase vacuum in the pipe to a much greater event to extract better, but that is where all the extra noise that type of pipe makes, it's proof of a pipe working well. If the baffle is made correctly the meg pipe can actually run about as well with the baffle in place as without, yet without much of the noise, the other straight can will not do that so well. It's all about working the sound waves there as much as possible, it greatly affects overlap flow to affect the jetting wildly. The tailpipe action being lower there means your jetting need not change nearly so much.

    Nobody hardly ever sets the valves on them either and it leads to valves burning when the clearances close up as many do, the compression falling off then again kills the engine revving in an absolute manner. It is pretty much worthless to rejet on an early DOHC unless you already know you have really good compression but nobody checks that. 170 psi is the number to shoot for.

    Hit close to that and I can give you some possible direction, there is no way with the engine in unknown condition.

    All that above is why you often find the bikes not revving out and choking at say 6000 rpm or worse, the carbs are off because the owner has fallen into the male testosterone thing we all tend to do. Or go bigger, these engines often do not respond well to it but they can under the right circumstances, they are just tighter than most.

  5. #5
    CB750 Guru amc49's Avatar
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    Missed one and aimed at post #1..............set the mixture screws at close to 2 to 2 1/2 turns, it helps to richen the idle based on the original EPA lean settings and it also helps with popping in the pipe at decel if the aircuts are not working well.

  6. #6
    CB750 Member mitchi's Avatar
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    Thank you Sir, much appreciated.

    Great information. Now off to check compression again, and readjust valves, if needed.

    Just not very good at valve adjusting.

    Once again, Thanks, Sir.

    And sorry for the threadjack John Luke.
    "If it ain't raining, I'm riding"

  7. #7
    CB750 Member John Luke's Avatar
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    Aw u good hoss. I think all it needed was for the carbs to be synced. Seem that the hanging revs are gone.

  8. #8
    CB750 Guru amc49's Avatar
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    Great. Carb sync can easily do that if one is too high compared to the others, it then flows enough that when the engine drops back to idle the high one hangs the idle high. Why? Because the high one exposes the small hole behind it to continue to feed fuel and the being high supplies the air, the idle then hangs because it has both to keep running. ALL slides must come down far enough to cut off those idle feed holes behind the slides or there is trouble.

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