Who has rebuilt cb750 carbs on a DOHC motor? I need you

Howdie y'all. I'm about to engage in my very first carb rebuild, no less than the 4 carbs on a cb750, DOHC. I'm trying to be in contact with someone who has been there and done that and I want to find you ahead of time. If you can answer questions for me and support me through this period I will be in your debt and am generally good at paying back. I'm an extremely motivated, passionate builder re-doing a cb750 of course. I have passable technical skill but I have never got into the nitty gritty of carb and motor work so would appreciate greatly, a reference. Thank you
 
I've done 3 or 4 sets in the past 6 months.....are you modifying the bike at all, or sticking with stock exhaust and airbox?
 
Nothing specific comes to mind as peculiar with these, so here are some generalizations.

1) "rebuilding" carbs is not so much a rebuild as it is a cleaning. Very often no parts need to be replaced except for gaskets. Not many parts go bad or wear appreciably except for floats, needles on gazillion mile or crazy vibration bikes, and diaphrams on CV carbs.
2) You will need to take your carbs 100% apart. In my experience, 1 in 100 people (I am too generous I know) actually understand what 100% means.
3) For the most part, aftermarket "rebuild kits" are an absolute waste of money, and the source of uncountable hours of frustration and confusion, not to mention money to pay someone to figure out how to get all the OEM parts back in. I had a bike in my shop a maybe a month ago - 1971 vintage - incredible museum piece. Very savvy owner could not get it to run properly. Carbs were IMACULATE! All I did was replace all the jets, needles, emulsion tubes, etc with oem parts - threw out all the "new" rebuild kit parts. Pretty easy - bike was stock down to the paper air filter so it was fairly easy to determine all the correct parts. Ran perfectly afterward. Took me longer to research and round up the parts than do the work.
4) You can soak your carbs - even completely stripped - in lemon juice, simple green, pinesol or whatever miracle juice you like, intern them in an ultra sonic cleaner for days or send them to NASA and it won't be enough by itself. I am not condemning any of these choices, simply stating that a) very likely it is not required, and b)it will not save you from meticulously inspecting and cleaning each and every tiny passageway one by one. If you are not up to doing this you are comprehensively wasting your time.
5) Most carbs are not all that complex, but a rack of 4 has quite a few parts. For the most part, there are only 1/4 as many parts as the four carbs are the same - mostly. Most sets have some peculiarities between them. Most of the differences revolve around the choke (or enrichers), fuel supply or mechanical assembly of the set and throttle actuation mechanism, so pay carefull attention to these areas BEFORE you take them apart. Occasionally they can be hard to figure out going back together even for someone very experienced. Good time to take some photos. One tactic is to divide the job into sections. Get the individual carbs separated and put all the loose bits in zip-locks. Then do each carb one at a time and keep all the parts for that carb in it's own baggy.
6) Many parts are brass. VERY SOFT, and VERY WEAK. The absolutely perfect and correct tool is essential. Get real comfortable with the notion of grinding flat blade screwdrivers for an absolutely flawless fit. Blades should fit with absolutely zero slop. You only have your first try removing jets to be successful. Damage the head of the jet and you sometimes have to be very clever indeed to not end up throwing out the whole carb body. If anything does not unscrew fairly easily, STOP! Heat the body up with a heat gun and try again.
7) Spray aerosol carb cleaner through all passageways. THERE IS NO SUBSTITUTE! Pay attention to the stream where it exits. Most of the time it exits in several places. You should always get a nice clean jet. Compare the exit jet to the other carbs. If one looks different figure out why. Avoid poking anything into any hole in any part. Only rarely is this actually necessary. If you think it is, soak the component in liquid carb cleaner (or miracle goo of your choice) overnight and try again. I think it is invaluable to learn how all the passageways of your carbs work. This will hugely improve your ability to clean them because you understand what need to happen when clean. It is much easier than it sounds - lots of stuff on the interweb. The most important thing to understand is that all the systems inside your carbs do something many people do not realize: They do not spit fuel into the main bore of your carb. They spit a mixture of fuel AND air into the main bore of your carb. As a rule, all the systems mix fuel with air within all those mysterious passageways BEFORE it emerges into the main bore where that mixture is mixed with a lot more air. Understanding where both the fuel and air have to go to accomplish this makes it a lot easier to KNOW you have actually cleaned everything.
8 ) Reassembly is usually pretty easy. Most needle and seat assemblies have needles with a spring loaded pin. This is a mechanical element that does not in any way interact with the fuel supply. Set the float height with the spring UN-compressed but the conical needle fully seated. Many carbs can not be held upside down to set the float height as the weight of the floats compresses the spring giving an incorrect height. Angle the carb so the seat is fully closed but the spring loaded pin is not compressed. In the main the float height is not crazy critical so don't agonize over the trickiness of making the measurement. Just get the spec and set the floats. 99% of carbs will have the operating arm horizontal, or parallel with the bowl gasket surface but there are exceptions. Be careful not to over tighten anything! The soft brass should go without saying, but I see a lot of distorted fuel bowls from over tightening screws. They will never seal if they get bent!

be patient, take lots of pics, and good luck!
 
I've done dozens of them. (kinda specialised in CB750/900 DOHC for a few years in the mid~late 80's)
One thing I will tell you, they will not work properly without stock airbox.
Honda couldn't get stock carbs to work without air box and they threw a load of money, engineers, carb specialists, etc at them.
The closest to getting them working 'properly' was Leon Moss of LEDAR RACING DEVELOPMENTS.
It involves re-drilling passageways and fitting air bleed tubes into float chamber.
You can get the bike to run but not clean pick up, cruise or even clean acceleration/top speed with 'pods' or bellmouths/trumpets (whatever you want to call them)
 
Damn,
Yes I've heard of this problem with the DOHC carbs. I've also heard from more than a few people who have found ways around this. Even if it just comes down to limiting air flow, that's the only difference with the airbox and pods right? From what I've read among the words of gents like yourself in the forums is that the "CD" carbs rely on some kind of vacuum action so actually need a restricted airflow to some extent and that's really all the airbox does...limits the air intake to a certain cfm right? I'm thinking of the airbox on my DOHC and I can't see any possible function it would serve other than to give an exactly moderated CFM. If this is the case, the problem has a solution, however illusive it may prove to be. I've read at least one account of a very excited individual who claims he got his carb/pods setup working great simply by restricting most of the pod with electrical tape. While I applaud his ingenuity and am excited for him, I will be more aesthetically minded and restrict the airflow using specially made plates inserted within the pod, out of sight rather than rock pods with tape on them. No thanks. I'm not doing everything I'm doing to make my dream bike then slapping tape on it. Anyways, what do you think Mr. DOHC vet, I'm going to give it a go restricting airflow in pods, do you think it may work relatively well if its really thoroughly tinkered with?
 
focusinprogress... Yes I am. Under a heightened awareness of how difficult it will be with a DOHC bike but encouraged by anecdotes of success...I'll be running the bike with some sort of modified pod system (likely just highly restricted but otherwise regular pods) and a very short, very low back pressure exhaust system to further complicate matters. I'm a masochist. However, I have purchased a DOHC carb modification kit from 6sigma that's designed specifically to balance modified exhaust and intake setups. I'm hoping that kit will smooth out the power curve wrinkles a little but I have a few other ideas and will keep researching.

Thanks Mobius, I actually am less intimidated now.
 
If you want to run pods then ditch those shit heap carbs, I put mikuni bst36ss carbs off a 86 gsxr 750. They work great with little tuning required. And they are still a CV carb so they are cheap and pretty reliable at any elevation, they have a nylon diaphragm instead of a big ass aluminum puck, so they work much better with pods. There is tons of information for the swap on the 1100f.net site.
 
crazypj said:
I've done dozens of them. (kinda specialised in CB750/900 DOHC for a few years in the mid~late 80's)
One thing I will tell you, they will not work properly without stock airbox.
Honda couldn't get stock carbs to work without air box and they threw a load of money, engineers, carb specialists, etc at them.
The closest to getting them working 'properly' was Leon Moss of LEDAR RACING DEVELOPMENTS.
It involves re-drilling passageways and fitting air bleed tubes into float chamber.
You can get the bike to run but not clean pick up, cruise or even clean acceleration/top speed with 'pods' or bellmouths/trumpets (whatever you want to call them)

I have to disagree here, and it's mostly due to more recent development in aftermarket availability. I tried like hell to get the stock carbs to run right with pods and a CycleX sidewinder exhaust. I could get the main jet dialed in easily, and could get the pilot jet pretty close....but the transition was nothing short of terrible no matter what I did. I hopped on the DOHC forums, and after some reading (everyone says you can't avoid using a dynojet kit due to the custom needles they include).....lesson learned. I'll finish this thought below in response to the OP


Dr. Success L. Pants said:
focusinprogress... Yes I am. Under a heightened awareness of how difficult it will be with a DOHC bike but encouraged by anecdotes of success...I'll be running the bike with some sort of modified pod system (likely just highly restricted but otherwise regular pods) and a very short, very low back pressure exhaust system to further complicate matters. I'm a masochist. However, I have purchased a DOHC carb modification kit from 6sigma that's designed specifically to balance modified exhaust and intake setups. I'm hoping that kit will smooth out the power curve wrinkles a little but I have a few other ideas and will keep researching.

Thanks Mobius, I actually am less intimidated now.

do yourself a favor and just spend the $90 on amazon for a dynojet kit. the needles they come with will immediately make you realize why you're spending the money. they have a heavy "step" in taper where the midrange would otherwise be impossible to smoothly dial in for pods/exhaust. They will not sell the needles separate from the kit, I have tried. In the end, I used the needles from the kit and drilled the slide openings with the supplied bit as suggested by their directions, but had to bump the pilot one size higher and the main two sizes higher than they supplied to really get it perfect.
 
there's no bike dyno's within an hour radius of here, and it wasn't personally my bike. the bike came to my non-running for a complete cafe racer type build with the box of parts the customer picked....I built it, got it running, and tuned it until it ran smoothly enough for street use.

That said, she ran AWESOME and really pulled great with no hiccups or flat spots. I can't confirm any increase in power or anything, but driveability certainly was not sacrificed.

I was not at all displeased with the power as far as the butt-dyno is concerned, and would trade a couple horsepower to not deal with the airbox. The look and sound with the pods and exhaust also certainly is easier on the eyes and ears as well.
 
Mikuni's off a GSX or GSF Suzuki are probably the easiest and best choice, you could probably get a decent set for not much more than the price of Dynojet kit.
No matter what you do to the stock carbs it's real hard to get the transitions right
 
Pods = waste of time, in my experience. Probably could do something with them but I'd rather be riding the bike and not spend all of time fucking with it. Velocity stacks from Steel Dragon Performance on the other hand SUCK ... in a really good way (not that way you perv). Only have butt dyno to back it up but the difference is night and day. Stacks with CycleX sidewinder = hold on tight.
 
Dr. Success L. Pants said:
Damn,
Yes I've heard of this problem with the DOHC carbs. I've also heard from more than a few people who have found ways around this. Even if it just comes down to limiting air flow, that's the only difference with the airbox and pods right? From what I've read among the words of gents like yourself in the forums is that the "CD" carbs rely on some kind of vacuum action so actually need a restricted airflow to some extent and that's really all the airbox does...limits the air intake to a certain cfm right? I'm thinking of the airbox on my DOHC and I can't see any possible function it would serve other than to give an exactly moderated CFM. If this is the case, the problem has a solution, however illusive it may prove to be. I've read at least one account of a very excited individual who claims he got his carb/pods setup working great simply by restricting most of the pod with electrical tape. While I applaud his ingenuity and am excited for him, I will be more aesthetically minded and restrict the airflow using specially made plates inserted within the pod, out of sight rather than rock pods with tape on them. No thanks. I'm not doing everything I'm doing to make my dream bike then slapping tape on it. Anyways, what do you think Mr. DOHC vet, I'm going to give it a go restricting airflow in pods, do you think it may work relatively well if its really thoroughly tinkered with?
You are going to end upp with less hp than stock with that approach,
And to add something, the airbox is not only designed to keep an even atmospheric vacuum, its also designed to reverberate intake backpulses so as to keep an even flow of mixture going into the combustion chamber. Without the correct length and shape of the airbox, you will have a lot of backpulsing resulting in poor fuel mixture as the same mix will move back and forth in the carb, upsetting the mixture and often leading to overly rich running.
Also restricting with sharp plates inside the pods is something of the worst you could do performance wise.
Google laminar flow.
 
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