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Carb swap on 79 DOHC

Hippie459MN

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Im sure this has been answered before but for the life of me I can find the info and im sure someone has answered it for me before but I was wondering, I can get a complete carb rack in great shape that are ready to go for a killer price off a SOHC CB750 (From a 77 if I remember right) and I would love to ditch these stupid CV carbs on my DOHC and I was wondering if the will work on the the DOHC or is the carb spacing and/or the intake manifolds different size? I just hate to buy them and get them only to find out that they wont work at all. Thanks. :)
 
Rubber manifolds wont fit, never tried the spacing. It would take some time to get the jetted right also. Save up for some new aftermarket carbs, Keihin CR's or Mikuni RS carbs. Well worth the money.
 
Kind of what I figured. Thinking of maybe giving the 2 carb set up a shot from CycleX but not sure. They told me they work very good on the DOHC. I have just about had enough of these CV carbs.
 
Has anyone got these cycle x twin carbs fitted and running well? a set would cost me 400 as opposed to 600 for a set of cr29's, i realise performance of the twin carb setup would maybe not be as great as the cr's but i do like the look of the twin carbs/trumpets poking out the sides (with frame mods)..Murrays carbs also does a set of twin mikunis but they dont have the 'look' of the cyclex kit as they are angled inside the frame.
Any one had experiance with either?
 
You can get the whole cycle-x kit for $400? Looks like they are $500 on their web site but I might try to piece it together cause I have 2 VM34 carbs already I could use. I would just need the intakes, cables, and thats about it. I need to figure out my carb situation this year for sure though. Im sick of fighting with these stupid CV carbs year after year. You have a link for the Murrays carb set up?
 
thats in pounds sorry ,both cycle x and murrays are roughly the same price, cycle x has cast manifold and murrays has a welded,if you type in vm34 cb750 in google it should come up with both sets..I think cycle x needs a modded frame, but when they are fitted they look the bollocks as they poke out the sides, whereas the murrays kit points in between the frame rails so no need for chopping anything.I have a cx500 whick i will be using murrays carbs, he seems to have them dialled in better than anything bar getting a custom dyno tune.He doesnt have a site but all the info can be found on various forums
 
Give up on the SOHC carbs on a DOHC, I looked pretty close and carb spacing issues are only the beginning of it. The later mechanical linkage (you say '77) will strike bottom of tank. The manifolds are an issue too.

Twin carbs on a DOHC are a quick 10 hp. loser. The engine is tuned for IR and not having it screws the tune up.

Dunno why so many have problems with the CVs, I think they work just fine and no trouble with them. of course many blame the carbs when engine condition is the problem, I used to buy and sell DOHC based on that odd fact. Lots of profit there. Most won't lift a finger to check compression then drive themselves utterly nuts working on the carbs. Waste of time.
 
My issue with these CVs on my bike are that they are old, wore out, and im running pods. Plus im sure my motor is a weee bit tired but never fails, every spring I end up having to screw with them, I cant get the bike to actually idle, ever... Im sure I will do a complete motor rebuild one of these winters but this year it wasnt in the cards cause I dumped a ton of money into a boat restoration. LoL
 
Hi mate, i've had cyclexchange Mikuni Twin Carb set up on my bike now for about a year. I bought them for the looks as my 82 CB900F is Cafe Racer style. I love them and i hate them. I think they look super cool but it has taken me a long time and a few hundred dollars in buying different size pilot jets and valve slides and needle shims etc to get the bike running good.They are way too rich when you get them. I had a lot of issues with the plugs fouling and i find that more fuel/air mix goes down one side of the twin manifold than the other but that is because i haven't modified my frame and they poke out the side therefore the carb mouth is pointing say more to cylinder two than cylinder one. But after saying all that the hard work is done now. I was sick of taking off air boxes etc. These Mikuni's are very easy to work on and fast. 20150927_110313 (1024x576) (1024x576).jpgIMG_4027 (1024x542).jpg
 
think i might just try fab up some manifolds and go with a set of vm's what did you settle on for carb settings?
 
20 slow jets.2.0 valve slide,200 large jets, needle second clip from top with shim, air screw 1 1/2 turns out. I think thats about it.
 
I forgot to mention i have pod filters as i couldn't get the bike to run well with the velo stacks they provide as part of the carb package. Also i have 4 into 2 pipes, also climate and altitude may play a part in your tuning.
 
Like I said higher up, all that work for 10 less hp.

As well, the jetting specs are worthless if you don't know the airjets used, that alone throws them in the trash. You need the main high speed air jet and idle air jet size too to be able to correlate fuel jetting. An air jet change of only 10 will shove fuel jet off as much as 5 sizes.

With 4-2 intake and exhaust, you have destroyed the highest wave tuning you have in those engines, the result in the 1-4 and 2-3 action. Why that carb setup so kills power solid as a rock. You've turned the engine into two Honda 350s with a combined 60 hp. power output there.

'...i find that more fuel/air mix goes down one side of the twin manifold than the other but that is because i haven't modified my frame and they poke out the side therefore the carb mouth is pointing say more to cylinder two than cylinder one.'

Really.............LOL. How long did that take? Could've told you that without running the setup a bit. One cylinder will ALWAYS pull differently from its' paired one due to the wonky firing order when you split them in that fashion.
 
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Hey mate i'm just being truthful to the other guy who asked about these carbs and manifolds.I'm sure you appreciate people that try to mod their bike and think outside the box, some you win some you lose.We all rely on (experts) for advice and even they get it wrong. Some people laugh if you fall down and some people help you up.But at the end of the day i still have my original carbs and air box if i want to swap over. If we didn't try anything new or different we would all be driving Volkswagen Beetles.
 
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If you fall a stranger will walk on by, a friend will help you up, a true friend will laugh and say "Been walking long?".

PS Don't miss the notes on CycleX. #1 not recommended for stock frames. #2 Made for SOHC but "will fit" DOHC.

(a lot of things will fit in the hole, most won't make it run well LOL)

Now I know your just talking about replacing the carbs, but your leaning towards major changes.

As AMC has pointed out in other posts, first you need to open the holes before trying to shove more fuel in. In other words, a better flowing exhaust is the place to start. After that would be work on the head, porting and polishing makes a world of difference along with some modern valve seats and guides. Then you toss on some bigger (total cfm) carbs, might need to change the fuel petcock and fuel lines, and don't forget the ignition!

In other words, think of the "entire" system.

Me, I'm an old mechanic. If it works don't fix it. But it hasn't always been so. I spent thousands on my first Camaro rebuilding it from the ground up even though it was in near perfect shape when I got it. So I know what drives people to re-engineer their bikes. Just remember to consider it as a whole, not focus on just one part.

And give us pics, we love pics!
 
Think out of the box? I live there. We raced American Motors cars back when you were considered an idiot for doing so. Had to make like half our parts as none available like electronic ignition. We made under 400 inch engines that would outpull big GM 454s in high gear, that's a considerable amount of humiliation.

Thinking outside the box can mean many things, one thing you CANNOT do there ever though is defy the physics of any thing or process, that will be where you fail miserably. Yet people insist on doing it all day long.

Installing a carb setup that detunes an engine heavily will not make an engine that does not run well to begin with suddenly one that runs perfectly. The stock twin 34s are laughably small, they should be at least 42 to equal the 30s you want to replace there. That's .320" twice of throttle bore you just tossed there.
 
Try messaging them but beings they havent been on here in over 4 years doubt you will hear from them. This post is 4 years old.
 
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