what is best replacement Carb ?

[5twins]

If by rusty you mean the chrome plated metal tops that come on the BS34s, you can swap alloy tops onto them from other model's carbs. Here's a set I just finished up as back-ups for my '83. The tops came from XS400 carbs .....

They can also be scrounged from XS750 triple carbs. These often have an angled side on them .....

 

[Team Junk]

I think a lot depends on the climate in your location.

The climate here in my part of Texas is pretty extreme . 2 degrees one day 80 5 days later. A big part of the fuel problem is the ethanol in the fuel. I can be be very damp here at times and the ethanol absorbs moisture and the combination attacks the metal in the carbs and the rubber. Then with the high heat it dries up into a nasty mess. Still haven't found the lost chord with mine but it does run. I plan to take my carbs off again and clean them once more.
As far as the best carbs it is hard to beat the stock bs38. By far the best gas milage vs vm34's for example. Unless you plan to be near redline most of the time the 38's provide better low low end grunt and keep their tune better.

 

[5twins]

Now that I have a BS34 equipped later model, I can't complain about them either. They seem very nice, run just as well as the 38s on my '78. Their advantages are that they are a bit more modern design, lighter, and parts for them are easier and cheaper to get.

 

[gggGary]

If by rusty you mean the chrome plated metal tops that come on the BS34s, you can swap alloy tops onto them from other model's carbs. Here's a set I just finished up as back-ups for my '83. The tops came from XS400 carbs .....

They can also be scrounged from XS750 triple carbs. These often have an angled side on them .....

To be clear the alloy tops from BS38s do NOT fit on the BS34s.

 

[jetmechmarty]

78/79 XS11 has BS34 with alloy tops. Four of them. No, you can't swap individual carb bodies.

 

[Grimly]

As far as the best carbs it is hard to beat the stock bs38. By far the best gas milage vs vm34's for example

Mine came to me with a pair of VM34s on it. After it sat for near a decade I finally got around to looking at it. The inside of the carbs were nasty with green flaky gluey junk. A good clean-out later and it was running. In subsequent outings, I regularly get close to 60mpg UK / 50mpg US, which is good enough for me.

 

[XSLeo]

When you aren't going to ride the bike for awhile you can turn off the fuel and drain the carbs. This should help prevent them getting plugged up.
Up her where I am we can't ride a; year. Snow gets a bit deep. One of the winterizing things we do is drain the carbs. Leave the tank full with fuel stabilizer. This helps prevent rust in the tank. It should keep the gas over te winter. If you don't trust it, you can drain it out in the spring and run it through your lawn mower.
One thing about the Kawasaki Ninja 500/EX500 carbs are they are a more modern carb than those used on the XS650. They are made from a different alloy that is more resistant to the ill effects of alcohol. They are loess apt to plug up over the winter if you don't drain them. I have a set of my 75. It runs very well and with the help of the thread about them they were easy to tune.
Leo

 

[arcticXS]

Here is a couple of pics of the RS/TM 34 flatslides on my 77 XS. The RH carb has the new setup I am working on, with a velocity stack and an UNI filter (which required a smaller battery box, and other mods) The LH carb has the oval and tapered K&N I have been using so far, that caused interference with the side covers.
The pic of the LH carb also shows how tight it is around the petcock, and also my rather busy and unsightly fuel hose arrangement. My tank has 2 petcocks, and the fuel barb is unfortunately on the outside of the LH carb. Apparently there is no option for a fuel connection between the carbs, which would be better. I will try to re-route the fuel hose when other jobs are sorted.

 

[grizld1]

Nice! Wish we could get those 34s on this side of the pond, but Mikuni America doesn't think we need or want them. You might consider a couple of things. First off, one petcock will move all the fuel your motor needs, and draining one side of the tank will leave only around 4 ounces of fuel in the other side, so you won't lose any significant range. Second, Tygon plastic line is more flexible and less bulky than the fabric reinforced rubber line in your photo. It will make the turn to the petcock without the brass fitting.

 

[arcticXS]

Nice! Wish we could get those 34s on this side of the pond, but Mikuni America doesn't think we need or want them. You might consider a couple of things. First off, one petcock will move all the fuel your motor needs, and draining one side of the tank will leave only around 4 ounces of fuel in the other side, so you won't lose any significant range. Second, Tygon plastic line is more flexible and less bulky than the fabric reinforced rubber line in your photo. It will make the turn to the petcock without the brass fitting.

The spigot on the carb is 8 mm, as is the hose from the brass Y- piece to the carb. The hoses between the Y- piece and the petcocks are 6 mm, or close to. And are in fact Tygon from Motion Pro. The 6-6-8 mm brass Y- piece is an eBay item from China.
I will keep in mind that 1 petcock will suffice, if I am unable to tidy it up using both.

 

[Jim]

And are in fact Tygon from Motion Pro.

Judging from your pic, that looks like braided nylon tubing. As Griz says, it's not very flexible due to the braiding. I'd question Motion Pro about that actually being Tygon tubing. The Tygon I've used can easily be doubled back on itself without kinking.

 

[jetmechmarty]

I'd question Motion Pro about that actually being Tygon tubing.

I'm a fan of the Motion Pro tubing, but it won't make tight bends. When you GTS tygon tubing, the Motion Pro Premium comes up, but I'm really not sure what it is. It claims to be Tygon.

 

[arcticXS]

Judging from your pic, that looks like braided nylon tubing. As Griz says, it's not very flexible due to the braiding. I'd question Motion Pro about that actually being Tygon tubing. The Tygon I've used can easily be doubled back on itself without kinking.

The nearest, 8 mm hose, is indeed generic, reinforced fuel hose. But the two 6 mm hoses are not. Pretty sure it is Tygon. Smooth, no reinforcement, and quite bendy.

 

[arcticXS]

Another thing, I ended up using straight (read non-angled) rubber manifolds from Allens (Mikuni dealer) in the UK. Neither the BS38 nor the BS34 manifolds fit correctly on the 34 flatslides. And I definitely wanted to avoid any vacuum leaks!
The drawback is that the carbs get a slight angle, which makes it necessary to modify the side covers. One mod almost always make 2 more mods necessary.......