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  1. #1
    CB750 Member jacob's Avatar
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    Cb750 Running wierd

    I recently restored a 1979 Cb750, had to put a new head gasket in, completely rebuilt carbs and carbs are synced stuff like that, anyway the engine runs great.
    I have a mac 4 into 1 exhaust and jets are at 105 main 75 idal, ( stock is 102 main 68 idal)
    But here is the problem, the plugs are white, so I go and adjust the A/F screw out to richen and it just runs worse, like hanging idal ( when the RMPs drop slowly) and the plugs are still white.
    This makes no sense to me because the engine runs so good. anyway im not trying to burn up a valve, so ill see if you guys could help.

    Any help will be much appreciated Thanks.

    *i did set the timing static (i dont have a timing light) but i figured this would not affect it.
    Last edited by jacob; 07-07-2019 at 05:38 PM.

  2. #2
    CB750 Guru amc49's Avatar
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    The jets that were 68 are not idle jets, they are low to mid throttle jets and called primary mains, the 102 are secondary mains. The idle jets are under the rubber plugs (MUST be tight and present) and not changeable if the carbs are original, they are pressed in. The official number would be around 38 on those.

    The issue with those carbs is that they clog with deposits faster than you can believe (even faster if ethanol used in your fuel, I've seen issues after bike only sat 2 weeks!) and most have to clean them 2 or 3 times to get them perfectly clean. Or give up on the bikes. The clogging makes them run like crap at lower throttles. The pressed in type idles are worse because you cannot pull the jets to clean them and they have cross passages in the upper tips that you cannot get to at all. The '81 and later models went back to screw in parts there and can be cleaned much easier.

    If you are still running the aircuts (under the small covers on left side of all carbs) then you may well have vacuum leaks and the carb boots commonly do it too from cracking when carbs get removed. New boots almost always needed. The hanging idle thing is vacuum leak or carbs open too much and not all the way shut. The aircuts can be bypassed to run better but the carbs pretty much need to be broken apart off the rack to do it easily. There is an accelerator pump in there like a car that should be squirting fuel when throttle is worked too.

    Get a timing light, you cannot set them static accurately as the mark is not a clear marker of where the timing goes (mag impulse, the timing changes with speed) and the advance is already partly working at idle speed to jink the timing off even more, you have no advance at zero rpm.

    Even running right new plugs stay white for a while, they are lean enough it takes a while to begin to color them up, use a -9 series plug instead of an -8.

    The engines are lean set for emissions and need the idle limiter caps removed to idle better if not already done, set the screws at 2-2 1/4 for best idle. The screws affect idle ONLY, you are wasting your time trying to use them for any rpm or running above idle alone. As soon as you touch that throttle the screws cease to have any effect.

    READ............invaluable.

    http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf

  3. #3
    CB750 Member jacob's Avatar
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    hey thanks for the response, so if you cannot remove the number 38 jets, are they just adjusted by the A/F screw on the bottom of carb?
    and how would your recommend cleaning the passages? I have sprayed caned carb cleaner up in the passage before.

    On my plugs i have over 100 miles on them and still white. if i move to a colder plug like a 9, would that fix it? because the A/F mix would still be off?

    and what is the limiter caps you talk about?

    also i looked for vacuum leaks and found in the throttle linkage on the number 1 carb there is a slight air leak, but not major,( you really gotta soak the spot before any rpm fluctuation).

    I was always under the impression that your idal jet adjusted from 0 - 1/8th throttle, so i would adjust the screw on the bottom to fix the lean mix while running. maybe i need to step up my 75 to a 78 and see if its better.

    i just dont get it because the bike runs really good.
    Thanks, Jacob

  4. #4
    CB750 Guru amc49's Avatar
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    If the bike were running good you wouldn't be here.

    Normally you would be right about 1/8 but on a Honda the primary jet controls most of the low throttle fuel until the slide begins to open. The 1/8 rule really applies to carbs with no throttle plate, the slide is it instead. These are also weird in that the primary and idle circuits are tied together and why the rubber plug. Look at the channel for primary and idle, they are joined by a drilled crosshole, the idle draws fuel from the primary until the primary then overtakes it to draw backwards. You are also missing the progression holes which feed fuel behind the throttle butterfly as it opens to expose them one by one, that passage can clog too and very hard to get clean as no way to check that one either. Those holes expose one by one as the throttle opens to richen while it opens up.

    The CV carbs HATE too rich and open the slides the most slightly lean and 75 is about it on primary even with a very good header which the MAC is not. CV carbs tend to auto rejet by the way they work, you do NOT rejet as much as with normal carbs or they go rich and then the bike does not run well. You aren't there yet but rest assured it's likely coming, everybody does it. Most headers on these run absolutely fine with no jetting up at all if the carbs are clean.

    The idle limiters are on the mixture screws and stop you from turning the screw more than maybe 3/4 turn max, yours may already be gone.

    Running the OEM airbox? You need to be, if not throwing away power there too.

    Best way to clean carbs is by soaking them and then high pressure air blower and again expect to do it more than once. If you don't deal with the aircuts you have major idle leaks if they are bad and possible if not done yet.

    All engines even ones that run bad appear to run good at higher speeds, the rpm covers up the human ear picking up on misfires at higher speeds unless they are very bad. And hard to argue against bad running when making the bigger power too.

    The 9 plug will color up a bit more and what I ran here in Texas 100% of the time with no issues.

  5. #5
    CB750 Member jacob's Avatar
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    Wow, this helps alot, I didn't realize I was tuning it up wrong.
    Anyways I pulled the carbs and im letting them soak in water with simple green (household degreeser) for a few days then gonna blow out with air compressor. May go ahead and do that a few times. Also i will drop the jetting to stock (102 and 68) and ill go from there.

    and yes im running air box, I tried the whole pod thing, just with the loss of power with CV carbs was not worth it.

  6. #6
    CB750 Guru amc49's Avatar
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    I absolutely would NOT be using any water based cleaner there at all. Best way to wreck the labyrinth seals at carb tops there is. Any soap used there if strong enough will be heavy alkaline and eats aluminum up quick. I once put a very expensive CB900 ported head in water based and almost ruined it after only 2 hours and got very paranoid to pull it out. It had already ruined valve shims and the corrosion was just beginning to pit aluminum. I got very lucky on that one.

    Solvent based cleaners are far better and do not hurt the metal parts, only rubber, which need to be removed if using them over. Most water base soaps will not touch hard varnish deposits hardly at all unless left in long enough to destroy other parts first.

    True soap is lye based and caustic soda, which is used in car engine parts vatting to clean iron and steel parts is a strong alkali soap as well, put aluminum part in that overnight and the next morning the part will be GONE. I went back to see what the link I gave used for cleaners and he mentions Pine-Sol or equivalent, if fresh that can be super rough on aluminum too, I've seen parts ruined sitting in it overnight, 15 minutes max if I ever use it at all. By then it will be etching the aluminum to a dark color already.

    Water based soap is harmless for minutes but time is the killer there. Throw a worthless piece of aluminum into any strong soap for a week and take a look at it, you won't do it again.

  7. #7
    CB750 New Member Jacob2's Avatar
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    (Same person, diff profile)
    I got everything cleaned out and should be reassembling tonight. That link with the information on the carb disassembly definitely helped out a lot
    I had some I had some questions about tuning the bike.
    So if the pressed in jet is supplying fuel at idle (no throttle) At what throttle position does the 68 and 102 jet start supplying fuel ?

  8. #8
    CB750 Guru amc49's Avatar
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    No exact position, more general. They overlap somewhat so one is over the other at some point to take over smoothly. As soon as the throttle plate moves in the slightest the primary (68) begins to try to flow and flows during the plate moving but slide not. The holes behind the plate open up to vacuum too and act as accelerator pumps since the throttle movement is so small it does not activate the true pump or primary for a few milliseconds. When the primary activates the more it flows the less the idle (pilot) jet does, the system is double ended to have one seesaw off the other. The secondary (102) begins to go up with enough air flowing under slide to activate it and the slide begins to open about the same time. The problem with CVs is that the secondary can be flowing with the slide slow to work due to airbox gone to pods or the engine draw low because of valves not set or dead worn out engine and then it goes too rich due to artificial conditions not warranted. It can be helpful to know that the 68 jet does NOT drop off at higher flows and rpm, it adds to the 102 to get total jet needed at higher loads. 1100 carbs drop the primary system entirely as the engine is big enough it doesn't need it but doing so bumps the jetting requirement for secondary to around 130 or so to make up for missing primary fuel flow. The smaller engine needs the primary as it doesn't make enough vacuum to pull the secondary into use fast enough. The 1100 carb has more holes behind the throttle plate to make up for missing primary flow too.

    With CV slides you never know exactly where the throttle slide is as it is NOT positively connected to your hand like a direct draw carb is. Why automatically jetting them up often kills them, CVs like to run slightly lean and the too rich then kills slide lift and then it richens even more. A good jet combination that I have found works well with pods is 75-115 but the header must flow well and WORK as in increasing vacuum in pipe correctly, so many do not do that well and just for looks. The engine must have good compression too with no valves leaking. Even then the bike will not fully open the slides 100% at top rpm in high gear, only maybe 85% and why the airbox is needed to open the last bit. Of course the airbox on means reduced jetting to compensate. Pods will pull out to maybe 8000-8600 rpm only but you are running at 120 mph there so no loss not doing it. Everywhere else the bike will run fine with bike pulling to easy 10,000 rpm but most will have trouble as the engines are typically NOT sealed 100% and then the problems begin for most people. That jet setting will be too rich if the engine is off and after a while of driving bike it will begin to miss at higher rpm getting on it from too rich. Part of it is from the weak OEM ignition too.

    There are TWO idle air bleeds to hit emissions; they both are open at pure idle but one kicks closed at higher rpm rpm decels to richen the decel enough to stop popping in exhaust pipe, at lower rpm it then opens back up to lean again. What is under the left side covers on the carbs.

    When you are at cruise throttle the slides generally are not open, all primary only.

  9. #9
    CB750 New Member Jacob2's Avatar
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    If at stock it runs a little bit lean what color plugs do I need to be looking for?

  10. #10
    CB750 Guru amc49's Avatar
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    They will stay white at first then slowly color brownish or tan. It takes a while. Look with a strong light up inside where the porcelain meets the steel shell, they color up in there first. One reason I ran the 9 plugs, they color up faster.

  11. #11
    CB750 Member jacob's Avatar
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    Click image for larger version. 

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    Here is a pic of the plug after 10 miles, its a grayish white, is this okay? I also have a 70 and 105 in currently.
    The bike does run a little better, less deceleration backfire.
    im probably gonna drop to stock jetting 68,102 because it does sputter at full throttle a little bit.


    Edit, just saw your post, I will ride more and check back on plugs.
    Last edited by jacob; Yesterday at 07:12 PM.

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