Mikuni BS replaceable with Mikuni VM on XS750? What?

M

manas

Been Around the Block
So I like to think that I understand the principles behind carburetors pretty well but will admit that once they're strapped on to the motorcycle it might as well be black magic that goes on in there.

Anyways, I was under the impression that the XS750 will only work properly with the stock Mikuni BS Mark 1 or 2 carbs until I stumbled upon this ancient looking page : http://www.650motorcycles.com/JimMitchell.html

But it looks like the VM36 works just as well. Does anyone have any experience with this? From my searching it seems as though Mikuni advertises the VMs for single cylinder applications. But I suppose it doesnt matter how many cylinders as long as you can sync them.

Why do I ask? Mainly curiosity. Also, it seems as though VMs are still manufactured or at least or on the market in far greater numbers than the old BS carbs, so that provides so convenience in case I ever break something or decide to switch.

Many thanks for any input..

-Manas
 
Carburetors in theory, all work the same...and they do..but all engines don't work the same. the carburetor needs then, to be set up for the engine. for instance, a 2 cycle engine tends to have reverse flow of air through the carburetor for a short moment, the reverses and pulls in the charge. This reversal causes changes in the mixture that needs to be compensated for, so the jets are different then a similar size four stroke engine. In addition, the 2 stroke has one intake per revolution, but a four stroke has only 1 intake per 2 revolutions. So on a similar size engine in displacement, the size of the carburetor needs to be different.

The length of the intake tract, the length and shape of the inlet to carburetor, all effect it. that is why, even though the carburetor itself is pretty simple, it needs to be setup for the particular engine and it's arrangement. If you use pods instead of stock filters, less restrictive exhaust, you change the nature of the air stream through the carburetor and it needs to be adjusted for it.

That is why a specific carburetor can work for one person, and not another with almost the same bike. Small differences in the intake and exhaust systems can change jetting and sizes needed.

A large carburetor for a given motor will work well at wide throttle settings, but have to slow an air flow at slow engine speeds to mix fuel well.

A small carburetor will mix fuel well at lower engine speeds, but hinder performance at large throttle openings.

Oddly, though harder to setup, a CV carburetor is much more adaptable to those varying air flows then a non CV unit. That's why so many mfgr's use them for the street bike. For racing or performance bikes, they tend to use non CV carbs for the slightly better full throttle applications, as racers and performance people will put up with poor idle and low rpm response.

there are other factor s too, but I heard the bell ring and this class is over...LOL

so the carbs are simple, the engines aren't.
 
mydlyfkryzis said:
.................. For racing or performance bikes, they tend to use non CV carbs for the slightly better full throttle applications, as racers and performance people will put up with poor idle and low rpm response.
Click to expand...

Come on now. I like my race bikes to run clean all the way down to 6 or 7000 RPM (small throttle low revs) :-)

On a CB175 we run them from say 6-11k revs and once it starts, it never sees lower revs on the track than 6000. We do like a clean roll on from almost closed throttle but the same bike on the street would rarely see those revs and would have to be clean at low speed in traffic. That is a lot harder to organize.
 
dual vm34s will work very well on the 650 and 750 good thru all the rpm range
 
Re: Mikuni BS replaceable with Mikuni VM on XS750? What?

CXMAN: besides being informative, that was entertaining to read. Thank you :) You say dual VM34s for an XS750, but I'm guessing you mean 3?
 
Re: Mikuni BS replaceable with Mikuni VM on XS750? What?

Maybe someday I'll try a VM34 on the XS, but for now its good info stored safely in my head. Thanks for the thread guys..
 
Re: Mikuni BS replaceable with Mikuni VM on XS750? What?

Also: stupid question but despite being called VMs are they also Constant Velocity designs? Or does the V stand for variable?
 
three 34's would be too much. maby 30's or 32's since they're feedig smaller cylenders then say 34's on a 650 twin.
 
VM34 on a big twin, Yes. On a 750 triple, probably a little large.

CV is a design of carb and it means Constant velocity because pressure differentials cause the slide to rise as required. Makes for much smoother operation on the street but with a slight delay compared to a well set up piston/slide carb.

Yamaha uses the same CV design in their EFI systems on R1s and R6s, so it does work and well.
 
34s work just fine and you can get at the adjustments on the as the vm34s can be had in rights and lefts

it came with 32s
 
YOU WILL LOSE bottom end with carbs that big on a cylinder that small. Smaller would be better. what about some TM28's? those might be best. have a look at the mikuni carb guide, its available everywhere online. they have the spread sheet to best choose the carb for your set up.ji
 
Re: Re: Mikuni BS replaceable with Mikuni VM on XS750? What?

ProSimex said:
YOU WILL LOSE bottom end with carbs that big on a cylinder that small. Smaller would be better. what about some TM28's? those might be best. have a look at the mikuni carb guide, its available everywhere online. they have the spread sheet to best choose the carb for your set up.ji
Click to expand...

I was just asking from a theoretical stand point. I'm completely new to the subject of carburetors. But this is all great knowledge. Now I'm on my way to downloading that Mikuni carb tuning manual that you're talking about. Is it available online in PDF form?
 
Re: Re: Re: Mikuni BS replaceable with Mikuni VM on XS750? What?

manas said:
I was just asking from a theoretical stand point. I'm completely new to the subject of carburetors. But this is all great knowledge. Now I'm on my way to downloading that Mikuni carb tuning manual that you're talking about. Is it available online in PDF form?
Click to expand...

And a note to people with short tempers : I will definitely google around for it.
 
it is a good thing to do a lot of research if you are not used to working with these carbs

there are a lot of self professing experts around who have never actually installed a set of carbs on

anything

check out what is around in commercial kits for your bike type that will give you some basic parameters

then decide what you want the carbs to do for your driving needs and running speeds and what other mods

you may be doing to the engine and bike and that will narrow down the choice for you
 
Re: Re: Mikuni BS replaceable with Mikuni VM on XS750? What?

cxman said:
it is a good thing to do a lot of research if you are not used to working with these carbs

there are a lot of self professing experts around who have never actually installed a set of carbs on

anything

check out what is around in commercial kits for your bike type that will give you some basic parameters

then decide what you want the carbs to do for your driving needs and running speeds and what other mods

you may be doing to the engine and bike and that will narrow down the choice for you
Click to expand...

For now, I aim to work with the stock carburetors. I'm a beginner and I know my limits : I aim to start small and gain some knowledge and experience. I'll wait for 'bigger' mods.

However, one modification that I am making is the air pods. I don't have much of a choice since I have no functional air box at the moment. Some people have pointed out that tuning a CV for pods is next to impossible and others have mentioned that it is doable. Since I don't have a choice I might as well be optimistic and try my best. I'm not looking for jaw dropping performance anyways. As long as I can tune the carbs to ensure a healthy engine that's neither too hot nor too dangerously rich...a little bit of loss in performance is fine by me.
 
the biggest issue with pods is to select ones that do not block or restrict the sensing port

so make sure that where the pod goes on the carb is the same size all the way back to the air filter

here this explains it if you start with good pods you will get it dialed in easier


http://cx500forum.com/index.php?/topic/17865-pods-plug-atmospheric-ports/
 
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