Cb750 Dual Carb Intake Design

*First off, I know that the companies currently making the vm34 kits have invested hours of r&d, and the easy route would be to just by a kit. My reasons for making my own are 1. My bike is already built and they probably won't fit. 2. I think it would be cool to make manifolds, if they work. 3. Why not try something different?

I've been thinking over making a dual VM34 carb set for my dohc cb750. I know there are a few companies currently making kits, both with very good feedback.
The two most popular kits are Murry's Carbs (sold by speedmotoco.com) and cyclexchange.net. Both kits use a tubular "y" design manifold (cyclexchange is cast, but tubular), but I've been considering an aluminum rounded-box manifold design similar to performance cars. While cars utilize one long tapered manifold I would make two. The carbs would sit directed as centered as possible, but the manifold would taper towards the further cylinder to increase air velocity in an effort to prevent the inner cylinders from running lean. Also, the manifold is wider near the outside cylinders to help prevent them from running rich because they are closer to the carbs. The outlet tubes to the cylinders would sit flush inside and have the welded connection area ground smooth/rounded.

I made a quick sketch of my idea, what do you think?



An aluminum race manifold:

no good it looses too much velocity in the open box
copy the ones that are already a proven success
the fact that you think making it wider near the outside cyls's will affect it running rich shows how little you know
the mixture is fixed once it leaves the carb
the picture you show of the multi cyl job looks like a blown manifold whole diff animal

boy do i like watching people copy my stuff

sorry :-[ he should just buy it, i mean really,srysly

well at least he will copy the old one i have a new better design coming out that makes more power and solved a few residual problems

xb33bsa said:

no good it looses too much velocity in the open box
copy the ones that are already a proven success
the fact that you think making it wider near the outside cyls's will affect it running rich shows how little you know
the mixture is fixed once it leaves the carb
the picture you show of the multi cyl job looks like a blown manifold whole diff animal

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I see. I know that the mixture is fixed coming from the carb. Believe it or not, I know the basic principals of how carburetors work. However I don't know very much about manifolds, which is why I posted. Are we only supposed to post here asking for input if we're experts?

I've read that the problem with longer manifolds was that the further cylinders ran lean, and one would have to jet larger to accommodate. I thought that was why most manifolds are larger near the closer cylinders, and tapered towards the rear.

I also planned to make the largest part of my manifold the same size as the tube used by the above companies, and have it taper smaller. Therefore there would actually be equal or slightly less "room" inside my manifolds than the produced versions.

well why not make it out of tubing then ?
just like the ones you can buy that have been proven to work well ?
they are short and efficient, why do you need your carbs at a crazy angle ?to be different?

cxman said:

boy do i like watching people copy my stuff

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If you missed it, I actually said at the top that I wanted to try something different. I also said that your products get very good feedback.
It was suggested that I copy your product, but I will not. On a future build, I very well may purchase your product in the beginning so I could make sure that it will fit with my aesthetic design. It looks like I would have to change my current battery box location and possibly tank, and it is just not worth it at this point in the build.
I tried to make this clear up front, but maybe I didn't. I have nothing but good things to say about your company or products, and I certainly don't intend to copy anything proprietary. Unfortunately, as you know, when working on older vehicles there is virtually nothing that hasn't been done in some way before. I'm just trying to put my spin on it.

xb33bsa said:

well why not make it out of tubing then ?
just like the ones you can buy that have been proven to work well ?
they are short and efficient, why do you need your carbs at a crazy angle ?to be different?

Click to expand...

Basically, to try something new that will fit the current bike configuration. I don't want to just try and steal someone else's work. I find joy in fabricating things. Sometimes it takes me hours to build something and it doesn't work, but I probably learned a bit by making it. I'm not a mechanic, engine guru, carb genius etc. nor am I in a rush to get things done. If I make manifolds that are unique, work and I save money that's a great day...

I had been toying with this sort of idea as well. Some other issues I have found searching for info, is that the fuel can come out of suspension when it is making its way into the cylinder. Automotive carburated intake manifolds are warmed by coolant which helps with that issue.
Also, look at tuning the runner length to rpm.

I am interested in seeing where this conversation leads.

cxman said:

boy do i like watching people copy my stuff

Click to expand...

Like you were the first to make these....

Heavies said:

I had been toying with this sort of idea as well. Some other issues I have found searching for info, is that the fuel can come out of suspension when it is making its way into the cylinder. Automotive carburated intake manifolds are warmed by coolant which helps with that issue.
Also, look at tuning the runner length to rpm.

I am interested in seeing where this conversation leads.

Click to expand...

I thought it was a interesting concept...

Unfortunately, I'm not sure that this conversation is going to lead anywhere good. I was hoping for some constructive criticism, much like your reply come to think of it. Instead I have someone telling me that I am copying him when I'm not, then someone telling me TO copy him, then the same person that is telling me to copy is agreeing with the person accusing me of copying and then again telling me to copy....This is all very confusing ;D

copying a proven intelligent design for your own use is one thing, its called being smart and a little humble
going to production to sell a copy is a dif kettle o fish
good luck with whatever you do

xb33bsa said:

copying a proven intelligent design for your own use is one thing, its called being smart and a little humble
going to production to sell a copy is a dif kettle o fish
good luck with whatever you do

Click to expand...

Thank you.

It seems like from the manufacturer's point of view, copying is not good at all...and no one said anything about going into production.

as i told you on the phone the other day designing a manifold is not a simple task

while your manifold allows for "packaging " in other words fits your bike and looks how you want

they will not flow well and will have a lot of reversion

the biggest problem you will have with them is icing making the carbs stick at high idle or worse in any kind of humidity the slides will freeze up

it took a lot of research and wet flow bench time to work out what is a simple looking but critical dimensioned intake and even at that

i have kept doing more work over the last year and it has payed off further so a new design is going into production

and no when you spend the kind of money it takes to design refine test and then market and inventory a product like this

copying is not a good thing and if it were a production issue it would be stopped i have intellectual property rights registered globally

that is not the issue the issue here is you are not copying the design you are making your own design and i wish you a lot of luck

cxman said:

as i told you on the phone the other day designing a manifold is not a simple task

while your manifold allows for "packaging " in other words fits your bike and looks how you want

they will not flow well and will have a lot of reversion

the biggest problem you will have with them is icing making the carbs stick at high idle or worse in any kind of humidity the slides will freeze up

it took a lot of research and wet flow bench time to work out what is a simple looking but critical dimensioned intake and even at that

i have kept doing more work over the last year and it has payed off further so a new design is going into production

and no when you spend the kind of money it takes to design refine test and then market and inventory a product like this

copying is not a good thing and if it were a production issue it would be stopped i have intellectual property rights registered globally

that is not the issue the issue here is you are not copying the design you are making your own design and i wish you a lot of luck

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Hi Murry, I didn't realize it was you. We did not speak on the phone, I did however email you for advise and we exchanged a few emails when you said just this.

Thank you for acknowledging that I am not trying to copy your work, but why did you accuse me in the first place?

Intellectual property rights on what?
It's not even almost a new idea, design or part.
Come off it a bit.

Any actual performance info, gains or tests?
Still don't get why anyone thinks these twin carb set ups are a good idea.
Plenty of real performance parts out there like smoothbores, FI or CR carbs so what's the point?
Some BS about being simple?

For the same or less one can have a rack of carbs that work.
And no need to fiddle with two chokes.

All this talk about R&D, but none of that is posted on the site selling them...

I wouldn't mind knowing the exact reason why this wouldn't work or hasn't worked.

Cars run single carburators with 2, 4, 6, 8+ cylinders. I had a straight 6 with dual single barrels, only difference in that is that the manifold was warmed by the exaust under it. It had long runners to the end cylinders and short shots to the inboard, also a plenum under the carbs, in my mind, similar to the OP's design. Is it the warming of the intake the key to this?

Heavies said:

I wouldn't mind knowing the exact reason why this wouldn't work or hasn't worked.

Cars run single carburators with 2, 4, 6, 8+ cylinders. I had a straight 6 with dual single barrels, only difference in that is that the manifold was warmed by the exaust under it. It had long runners to the end cylinders and short shots to the inboard, also a plenum under the carbs, in my mind, similar to the OP's design. Is it the warming of the intake the key to this?

Click to expand...

the difference is a motorcycle engine is in an extreme state of high perf tune compared to the examples you refer to
these old jap motors are putting out close to 100 hp to the liter, the 2 valve ones, the 4 valvers are all higher than that
you cannot get that without really efficient breathing
if it was just as good to have less than one carb per cylinder the mfg's would have done it in the first place fer fucks sakes
think of the money they would have saved,you dont think they researched it ? hell yes they did
for the average joe or jill,it really borders on idiotic to not run the oem carb setup and airbox

nick

what confused me is the same day in fact one half hr later than we exchanged emails i got a call from a guy i thought was you

talking about the same sort of issue he wanted to get just the carbs and cable and filter so i wil now tell you the same as i spoke to him

if you want build your manifolds and send them to me i will install them on my test bike and run a set of carbs on them

and see how close i can get them for you in a hour or so keeping in mind that i have about every jet,slide.jet needle.pilot ect ect

and a air fuel meter on my test bike

and then i can ship the carbs and manifolds back to you so at least you have a running start and drive able bike to get tweaking with

as your bike and mine are not the same and i wont have free time to run road tests as well