BS34 Jetting

Hey guys, I'm using one of these carbs on an xvs650. I've heard that the xvs650 is quite similar to the xs. Is it reasonable to assume that since they are both 650's that I could jet this the same as if it were on an xs? If so, can someone give me a good starting point? I'm running a vent stack on the carb with no real air restriction, and fairly short drag pipes. If someone can tell me what sizes I need for all the jets I'd appreciate it. I'm not sure what this carb was used for in the past and I'd like to go ahead and replace all jets to be sure I have what I need installed. Thanks.

Can someone at least give me the jets one would start with if using these carbs on an xs with the same air restrictions? I tries a search with no luck. Also, does anyone know the diameter of the fuel t-fitting between carbs? I need to find a 90 replacement.

Here you go.



Note the BS34s came with brass floats in 80 then changed to foam floats in 81. The 27.3mm height is for the brass.

Anytime you have a BS34 open remove the float valve from the carb body and check the screen and o-ring, the o-ring normally needs to be replaced.

The taper jet only has one clip position, shimming it with a small washer helps richen it a bit and that cures a normal flat spot just off idle.

BS34s were used on a lot of bikes and they are not all the same body, did yours actually come from an XS650?

Mine have the brass floats, so they must be from 1980. These came off of a 79' xs that I used to own. It was my first bike, and I thought it would be cool to use the carb from it on this bike, especially since the new one is a Yamaha 650 as well. I wish I had just bought a new flat slide as I've had a lot of trouble making this carb work. At this point my intake is made, I've adapted the carb to work with the TPS, and I've finally located a replacement diaphragm for the slide so this is the carb I'm using.

Thanks for the chart. I'll make sure to get all the stock size jets that I need, but can you suggest a good starting point on the main and pilot jets? On an xs with these carbs, vent stacks, and drag pipes, what is the typical starting point for main jets, pilot jets, and mix screw?

I would start with the stock jetting. And see what plug chops look like. Odds are good the cam profile is designed to get the HP and torque down low. Short "dump" pipes probably won't flow any better than the stock muffler. Yeah hard to swallow but exhaust is not simple stuff, long fairly small head pipes opening into a cone inside a muffler flow pretty damned good. So try the stock jets see which way you need to go by looking at the plugs. An oddly shaped intake can throw jetting off too so you are pretty much the test pilot on this set up. All the jetting is readily available so going up and down shouldn't be that hard. Are you running one carb or two?

The usual stating point with BS34s and mods is stock jetting. The changes that work can be all over the board. A foam filter or some restriction in front of the carb will probably be needed, CV carbs don't usually like a completely unrestricted intake. The slides need some vacuum differential to operate.

Thanks again for the info. I'm using a single carb. Should be ready to try and fire it in the next few days.

What type of foam can be used in the stack?

UNI filter foam comes to mind. Or chop up something from the local lawn and garden filter selection.

Not to butt in but; Are you sure the carbs are BS 34's? 79' XS 650's came OEM with BS 38's Maybe post a pic of the carbs? Would not hurt to check the jetting and pass that info along as well. Carb boots could be a fitment issue too? I'd be surprised if they fit with the complex angling fit of the XS 650. Apart from all that It may not be reasonable to assume that a single CV carburetor of a matched pair supplying the XS 650 could do as well on it's own to properly supply both jugs of the V twin 650 of the Star by itself?

ie. It would not be reasonable to assume that single carburetor of a set of two, either the BS 34 or 38 variety could well to the job
properly for a XVS 650 V Twin. Blue

bluebikerblan said:

Not to butt in but; Are you sure the carbs are BS 34's? 79' XS 650's came OEM with BS 38's Maybe post a pic of the carbs? Would not hurt to check the jetting and pass that info along as well. Carb boots could be a fitment issue too? I'd be surprised if they fit with the complex angling fit of the XS 650. Apart from all that It may not be reasonable to assume that a single CV carburetor of a matched pair supplying the XS 650 could do as well on it's own to properly supply both jugs of the V twin 650 of the Star by itself?

ie. It would not be reasonable to assume that single carburetor of a set of two, either the BS 34 or 38 variety could well to the job
properly for a XVS 650 V Twin. Blue

Click to expand...

Actually, for the longest time, I thought what I had was a BS38. *It was only after I started measuring the diaphragm that I realized I had a 34. *After finding a few pic of BS34's on the net, I'm positive that's what I've got.

The original carbs on he XVS were a pair of downdraft 34mm carbs. *Not sure what model or make they are, but I don't see why one 34mm carb wouldn't do the same job. *The cylinders fire opposite of each other, so it's not like they are fighting for fuel or air. *I'm not saying that I can't be wrong, I don't know, but the above is the reason I assumed it would work.

This morning I was finally able to get the intake sealed to the engine, and the bike fired right up. *I've only run the bike a few short times to test things, but it seems to be running very nicely, with good throttle response. *Hopefully over the next few days I can dial it in a bit more and take it for a test ride.

Here is a link to some pics of the carb, manifold, and bike. *It's far from being finished, but so close to being rideable. *I'd love to hear any opinions anyone has.

http://www.flickr.com/photos/chopperdoug

Thanks for the help guys.

Edit: Sorry for the *'s, I have to copy and paste this on my iPad or it crashes. I don't know why it adds the *'s when I do that, but it does.

I'm with him 1 carb running two pots is common all over MC'ing. Good on you doing something different. If you can figure out a way getting a dome shaped filter out at the end of the velocity stack should help the carb work to it's potential. Bike looks good there's going to be a lot more bobbing of metric v-twins.

Well guys I'll check to the power on this one for apart from running a Mikuni HS 40 on my 90 Fatboy for years and years, I wouldn't have thought a single carb of a pair BS Series CV's would not have matched up well to the needs of the 650 V twin. I have little to add that might be relevant re jetting. So, I'll butt back out and follow with interest with hopes of learning. Best of luck! Blue

I was doing some research and came across your post. Your XVS is looking pretty damn good! Nice work on the manifold.