So, in another thread, we touched on the differences between our CV carbs and the VM series round slide mechanical carbs. What exactly is the difference between the two? What are the pros and cons of round slides versus flat slides? Would there be a benefit to switching at all? I'm basically looking for as much info as possible. I love learning.

Thanks!

Just want to bring it up (since it's my fault for using the wrong terminology), the shape of the slides are not what we are discussing here. It is the differences in constant velocity (or CV) carbs to mechanical operation carbs (such as Mikuni VM style carbs).

The GS500 carbs are CV carbs but with flat slides. All of the GS450's used CV carbs as well but had round slides.

Here is a good link for general carb operation:

http://www.dotheton.com/forum/index.php?topic=16745.0

And here's some info from my personal experience...... my GR650 came stock with BS36 carbs. They were just so old and finding replacement parts were a real hassle. When I was looking into replacement carbs, I naturally assumed VM36's were the ticket - but it was pointed out that VM34's were the size that I needed. I was told that CV carbs are generally larger to help with the airflow into the head.

But back to CV carbs in general: the vacuum created by the motor doesn't "push" the slide up like people think. The vacuum draws air into the mouth of the carbs, along with the holes around the edge. That oval hole at the 12 o'clock position on GS carbs? Air getting sucked into that hole is what makes the slides go up. Which helps to explain why gusts of wind can make a GS motor stumble. Also why cheap Emgo pod filters are useless - the rubber sleeve on Emgo filters literally blocks off that oval hole (and the other venturi around the carb mouth).

On CV carbs - when the throttle is twisted, the cable opens up the butterfly valves, which allows air to be drawn into the mouth of the carbs. If the motor is spinning at 3k rpm but the butterfly valves are wide open, the airflow is only exerting enough force to open the slides up to ALLOW the motor to run 3000 rpm. The engine will rev higher as more gas and air is taken in though, and therefore even more air enters the carbs / making the slides go up. Eventually the motor will suck in so much air that the slides will be all the way up (and the engine is red-lining). It's all fairly simple when you look at it. You control the butterfly which allows air in, which controls the slides, which controls the air / fuel mixture entering the engine.

Hopefully after reading some of this, there is a greater understanding of why a vacuum leak anywhere near the carbs causes a big problem.....

So, if I'm understanding this correctly, CV carbs are based around air velocity and the pressure differentials that it creates. I assume mechanical carbs rely on throttle position (the butterfly valves) for their operation? If that's the case, then mechanical carbs should offer all of the mixture right when you open the throttle, as opposed to CV carbs, which must wind their way up to full blast. This would then mean that, generally speaking, mechanical carbs offer better power, but CV carbs offer a smoother feeling power band, correct?

And thanks again. Like I said above, I enjoy learning things like this.

i dont trust those tricksy butterflies!! .. flittering around the place all innocent like!! .. then WHAMMO!! 



--- i posted this so i could easier keep track of the thread.. because i am interested... and if i 'notify' it .. the board sends me an email every post!! .. which is a pain.... ---

.. still dont trust those butterflies! 

.. and so i dont trash the thread this early... for BigRich below post... voodoo AND butterflies!!??... now you're a man that like to live on the edge!! 

When I talk about mechanical carbs, I specifically mean aftermarket Mikuni VM series btw......

You're close in thinking about VM carbs. They don't have butterfly valves - the throttle cable is physically attached to the slide itself. You twist the throttle = directly pulling up on the slide. And yes, CV carbs are generally smoother in operation than mechanical carbs.

The biggest problem with VM carbs? If you're cruising along around the idle position and crack the throttle open, the slides shoot up right NOW. But the motor isn't spinning fast enough to efficiently use the sudden burst of air / gas mix, and bogs down. I'll post more later tonight.

And Janx - good to have you aboard on this thread! I know you're interested in the voodoo magic of carbs......

Okay. I just read through that carb theory post. So, really the only difference is how the SLIDES operate? On mechanical, they go up with throttle position, whereas on CV they go up due to pressure differentials/vacuum. That's making a lot more sense now.

Edit: You posted it as I was writing lol.

And Janx... I don't even have the words right now lol.

Pesky butterflies.

Janx: Ha!

I said earlier about how VM carbs can bog down at WOT - but really even CV carbs can do the same thing. But it's much easier to bog down mechanical carbs. With little bit of experience with VM carbs, it is possible to get a smooth throttle response. Since VM carbs don't rely as much on airflow as CV carbs, they tend to be easier to tune after intake modifications. How many people have installed a K&N lunchbox on GS500 carbs and experienced that "cross wind stumble"? Well, that cross wind is affecting the slides in stock carbs. On VM carbs, a cross wind will just mess with your air / fuel mixture a little bit - it's gotta be a helluva wind to push the slides up.

In my own experience, VM34'S were much easier to tune than the stock BS36's (on my GR650). To change the main jet, all I had to do was take the float bowl drain out - the main jet is right above the drain. The pilot jet needed the float bowl removed completely though. The needles are notched, and the cap on top of the carb unscrewed as a unit (unlike the GS500 carbs where there are screws holding the top on). The air / fuel mixture screw is on the right side ofthe carb body, and the idle set screw is on the left OR right side of the carb (fdepending on whether it is the left or right carb).

Oh yeah - it is possible to have each carb set at a different idle position. But they can be set to the same height using the bottom end of a drill bit (the smooth end that is held by the drill). And unlike CV carbs that need to be synchronized using a vacuum gauge, VM carbs are sync'd by making sure the slides go up at the exact same time. With the engine off, place a finger in the mouth of each carb until they touch the slides. Twist the throttle gently - both slides should begin to move at the same time. If they don't, there is a bolt on top of the carb that the cable passes thru - adjusting the height of that bolt synchronizes the slides.