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Hey guys, so this might have been covered, if so I'm very sorry...my searching skills are have lost today.
Can I replace a pair of BS34 Mikunis for VM carbs as long as the VMs are 34mm (same as my CVs) ?
I ask because I was looking to get a pair of round slide carbs, and noticed the ones that are sold on the bay as snow mobile carbs are a hell of a lot cheaper than the ones labeled motorcycle carbs. They look the same to me in pictures but I'd rather know if they will work before spending the little cash I have atm.
You use a smaller slide carb than a CV.
In my experience go about 6mm smaller.
34mm CV is about equivalent to 26~28mm slide, smaller carb makes jetting much easier as venturi velocity is higher and works better
The only time to go same size is racing where your near max rpm all the time
BS34 is 34mm at the outlet but has a bore of around 29mm. If you replace the CV type carbs with regular VM, you will probably need much smaller number carbs.
Compare, for example GT750 Suzuki - 32mm VM type or BS40. CB350 CV carbs c/w 24mm slide carbs on the CL
Snowmobile carbs are cheap but the jets all have to be changed to run them on a bike plus they usually have water separators that need to be replaced with drain plugs. Not a problem if you know how to jet a carb.
I'm really curious about this only because I've been reading and have seen lots mention that CVs are not well suited for use with PODs instead of an air box. I haven't noticed any problems, but then again I am a novice rider and this is my first bike. I got the bike with a trashed air box, liked the way PODs looked and so installed them first thing. I've only ridden with the PODs and CVs, so I might not be able to tell the difference.
That being said, when referring to bore size, I assumed it was the size of the throat (?) that connects to the engine itself. Am I wrong in my assumption? If so how can I tell if the carbs will fit the engine itself? Is it pretty standard?
Also, when you say water separators should be replaced with drain plugs, are you talking about the brass screw at the bottom of the float bowl that alow you to drain gas from the carbs when storing the bike for winter for example?
I'm not too afraid of jetting the carbs as I spent most of last summer learning and dialing in the carbs I have on the bike. I've gotten it to the point where I'm pretty confident my tuning was correct, but as mentioned earlier I am a novice.
Cv carbs rely on the intake because of the butterfly valves controlled by the throttle. You twist the throttle, butterfly valves open which creates more vacuum, then the "smooth" air flow from the airbox lifts the slide smoothly. Pods allow more air in which interrupts that smooth air flow. Depending on the aerodynamics of the bike (and you on it) along with cross breezes, it can make cv carbs a nightmare to tune. Not always though.......
The slide diameter is what you need to know in referring to replacing the carbs. BS34's have a 34mm slide, VM28's have 28mm slides, etc.
Lots of good in[ut so far. Actually on a BS series carb the outlet where it goes to the motor is the quoted size but it has no relevance to teh flow rate or Effective size of the carb. Back to that BS40 on a Suzy 7 1/2. It's a BS40 and the outlet is 40mm in diameter. Where the slide goes up and down the floor is raised, the slide never gets out of the way and the sides are pinched in. The effective area is around 32mm. So it's called a BS40 and it's actually a 32mm carb.
On a BS34, the effective diameter (I have not measured one) would be closer to 28mm.
The slide diameter may be the same size as the carb says but that would be unusual. VM26 slide is 30mm, Vm28 slide is 30mm diameter, VM36 is 38mm. On keihins, it's more of a crap shoot because the older bikes feature figure 8 oval bores, just to make it hard to work out the nominal size.
So a PW26 on a CB77 has a 26mm slide and so does a PW28 from a SOHC CB750. The difference is the height and width of that oval bore. It's taller on a 750 and the effective diameter goes up slightly to a nominal 28mm.
So back to your question. You would want 28 or 30mm carbs, but if you have it running well, why bother? What's that old saying about things not being broke.....
Teaser, that'll take a bit to soak in, but I thank you for the awesome info.
I also took a look on Mikuni's site but did not find anything on the BS34s, so maybe I'm missing something, but I don't think they have the info I'm looking for there.
Not exactly what your asking, but youll need to know... Most of those snowmobile carbs are set up for two stroke motors. Youll need to replace the emulsion tubes and jets.
All I was trying to explain is that the "number" is sometimes teh slide diameter, but not always and it's usually more or less the nominal bore size (main part of the carb).
Yes that brass nut on the bowl is a drain on snowmobiles with a 4" length of blocked off tube. It collects water so that when it freezes, it doesn't ruin the carbs.
And yes, the main jets always need changing, pilot jets sometimes, slides occasionally, needles often and needle jets more than likely.
As I mentioned earlier, If you have it running well, it's not a great idea to swap carbs unless you like a challenge.
Cool, thanks all for the great info guys! I decided against the snow mobile carbs only because at first glance it seemed a great find. Seems like it might be more work than it's worth. Again, thanks!
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