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  1. #1
    CB750 Member yooper's Avatar
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    Jetting for 1976 CB750 K6 with PODS

    I apologize as I'm sure this has been asked before but I was unable to find it with the search bar.

    I have my carbs pulled apart (along with the rest of my bike) and want to put the correct sized jets in while it's easy.
    Any ideas on what jet sizes I should use?

    I have independent pod filters and a 4into2 exhaust.

    If there's any other information you need to help please let me know.

  2. #2
    CB750 Guru amc49's Avatar
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    You need to find out for yourself. There is NO ideal jetting for any setup unless they use the exact same pipe and even engine condition can change it. Even your location in the US will change it slightly.

    I'm sure somebody else will be along to say more.

  3. #3
    CB750 Member yooper's Avatar
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    Got it thanks!
    And this is pretty much based off of if its running rich or lean at idle and medium-high load right?
    If you have any suggestions for literature on this subject send it my way.

  4. #4
    CB750 Guru amc49's Avatar
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    You realize you are asking me for a 1000 page book right? I took 45 years to amass what I personally know about the subject.

    You don't generally ever rejet for idle, the idle is adjustable enough with the pilot screw and why most Hondas use the same or virtually identical idle jet over a wide stream of engine sizes. 35, 38, or 40 and 40 is often too big, they are all close to the same jet. High perf parts do not generally affect idle as the actual air entering the engine is so small in volume. Usually another half turn on idle mixture to richen and you are there but you need to know if that means further open or closed, there are idle systems that are backwards of others.

    I have a 550F that has pods and a much more efficient muffler on the header and only went from say 120 to 130 on main alone and it runs perfect. Sometimes you need to lift the needle up one step to help mid range after you do the main. I rarely do here in Texas, it may be needed if up north where air is generally colder.

    If anything runs richer after engine changes but before the rejetting you DECREASED power there with your mods. Happens all the time. Or engine has something wrong with it. A non-OEM 4-2 can do that if it uses a crap design internal baffle like some Chinese stuff uses. Pods can do it if the engine really liked the tube extension from the carbs to the airbox, the early DOHC does that. The pods shorten the effective velocity stack and then engine tune goes off. So then people are convinced they need the jet (because 'everybody else says to do it'), add it and then bike runs like crap when it wanted LESS fuel not more. Then not reading plugs leads to adding even more fuel because owner cannot grasp the reverse idea and bike pretty soon for sale with carbs off of it. I've gotten two free DOHCs that way. Both running fine 30 minutes later when they would even start before. Owners totally lost and plugs fouled out. Bought a $1000 new RM Suzuki once for $300 that wouldn't run and maybe 30 minutes on it, Suzuki made an unannounced factory needle jet change and the guy modded by changing the expansion chamber and jetted up and bike refused to run. I found the running change and dropped main jet DOWN about 20 numbers and bike instantly ran to 10,000+ rpm after a plug change. Sold it for $800 4 days later.

    You need to learn to read plugs, the only indicator you have of whether you are helping or messing things up worse.
    Last edited by amc49; 05-15-2020 at 12:31 AM.

  5. #5
    CB750 Guru amc49's Avatar
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    Still waiting on that guy (dirtdigger) to show up, he knows far more than me about SOHC mods. If he differs in any way from me listen to him.

  6. #6
    CB750 Member yooper's Avatar
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    If there is a book on the subject, I'm interested. I think you saying that it's more of an experience thing though.
    I'll look up some spark plug diagrams. I'm know some basics to adjusting oil/fuel ratio on a two stroke but I'm sure it differs when diagnosing AFR.'m

    Just curious, how difficult is it to adjust spark timing and cam timing. I'd assume you could move the points for spark and cam timing chain for cams, but there wouldn't really be a way to increase dwell time without buying new cams right?

    Can I ask how you came to acquire so much knowledge on the subject? Has it been a hobby for many years or are you a mechanic or engineer or something?

  7. #7
    Moderator dirtdigger's Avatar
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    Yes this question gets asked several times a week...just wander through the sohc section and you will see how often. Put the carbs together totally stock so you have a base to start from. stock jet size, needle position, float level etc. Get your ignition system in good working order first, adjusted or new, timing and dwell set, verify advancer is working, new plugs. Verify engine is mechanically sound....good compression, timing chain adjusted, sync the carbs. Ride it see how it runs. If you have a running issue, start closing the choke while riding where you have the issue....if it gets worse as you slowly close the choke it is already rich, if it gets better it is lean. I always do full throttle first to establish main jet size then work on the needle position. You will probably end up in the 110 to 115 area on the jets depending on how the exhaust works and how restrictive the pods are.

  8. #8
    CB750 Guru amc49's Avatar
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    Rock solid advice all of it.

    Myself, I worked on all kinds of engines and have rebuilt so many I cannot count. Car and bike 2 stroke and 4. Hotrodding car from the very basic to pro stock 1500+ hp and over 200 mph. Our family garage built lots of drag race engines. Cigarette boat racing too. I started out on 2 stroke engines and porting them. We raced AMC AMX race cars and had to learn to make many things ourselves that others could buy on Chevrolet or Ford stuff. I worked on those as well, GM 454 and SBC so many I can't count and Ford Cleveland and Mopar 426 and 440. I worked on a LOT of carburetors.

    Use both books and the experience and what I did, I read far more than most people ever do and how I learned to do literally anything. Engines are engines whether on car or bike, the basics are the same. The difference is in the details.

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