72 BS38 Carbs

[Smuggy]

Hi,

So my current carbs are in desperate need of a cleanup, rebuild and general overhaul of the internals. It's looking like I will also need to be replacing the carb joints, and I was thinking, is it not cheaper/easier to just replace them with something a little newer? I know this question has been asked, but I see most of the threads are from 8+ years ago.
So I have seen people mention VM34s as a solid replacement for the stock BS38s. I have also seen mentions of Lectron's and Keihin FCRs being great but expensive. Then ofcourse their is the TM34 from Topham/Allan's (depending where you are in the world) which looks to be an amazing bit of kit. Not sure if there is ever going to be a choice everyone agrees on, but hoping for some advice based on what I am looking for. I just want something that is going to be super simple to tune, easily accessible parts and reliable. Performance gains are an added bonus but not something as a priority for me. Should I stick with my BS38s, refurbish and rebuild them with as new a components as I can get my hands on? This of course allows me to keep my stock airboxes, all of the great guides on here for tuning them in the tech section as well as having Mailman's meticulously documented testing and tuning on his XS2 - but also means increasingly difficult to source parts. Are people that made the swap to VMs/TMs/Lectrons/Keihins wishing they had made the move years ago?

 

[DogBunny]

One option is to use later BS38s. 78-79 are arguably the best, but 76-77 are good too. These are linked carbs that use a single throttle cable, a vast improvement over dual cables. You can still use your old throttle sleeve and one of your old throttle cables. You will also still use the style of joint that you already have, the straight 70-72 type. Getting the linked carbs on and off the bike will be a little different. You will have to remove carbs from the the joints, then remove the joints from the engine to get to get the carbs out. Reverse sequence to put carbs on. You will be able to re-use your stock air boxes.

One more thing. You will have to remove the carb cap "butterfly". This is the first thing you take off when removing linked carbs from a 70-73 frame, and the last thing you put on when installing them.

 

[Mailman]

One option is to use later BS38s. 78-79 are arguably the best, but 76-77 are good too. These are linked carbs that use a single throttle cable, a vast improvement over dual cables. You can still use your old throttle sleeve and one of your old throttle cables. You will also still use the style of joint that you already have, the straight 70-72 type. Getting the linked carbs on and off the bike will be a little different. You will have to remove carbs from the the joints, then remove the joints from the engine to get to get the carbs out. Reverse sequence to put carbs on. You will be able to re-use your stock air boxes.

I never realized that the later linked carburetors would fit the early bikes, very interesting!

 

[5twins]

I thought the early intake manifolds were angled differently? The early manifolds have just one part number. The same manifold is used on both sides. The later ones have a right and left manifold.

 

[Smuggy]

I was also under the impression that you would need to use angled manifolds, but then stumbled across this thread where CalsXS does some work fitting the linked carbs with the straight intake manifolds. The main thing I deduced was that the carbs will be tilted forwards slightly, Leo summed it up here:

The straight holders hold the carbs straight out from engine. With the engine tilted forward this tips the back of the carbs higher than the front.
On the later bikes with angled carb holders the angle holds the carbs level.
Using angled holders moves the carbs a bit and they wont line up with stock air boxes. Not an issue if your using pod filters.
On the side covers this change may move the carbs enough so the sides covers touch the holders or carbs a bit. This may require a bit of modification to the covers.
If you have both a set of straight and angled holders I would try your carbs using both sets. This way you can see which set holds the carbs in the right place so the covers clear the carbs.
This change in tilt on the carbs probably won't be an issue. Just be aware it might have an effect on float height adjustment.
Leo

Here is the thread if you are interested in the pics/findings:
http://www.xs650.com/threads/72-bike-76-motor-78-carbs-which-boots-and-filters.38759/

 

[DogBunny]

Not a great pic of them, but I have been using a set of linked 76-77 carbs on my Super RustBucket ever since I got it running. This is a 1973 frame, the same as all 70-73 frames. I very briefly mentioned this in passing in my "1973 Super RustBucket Ressurection" thread.

I went into the use of linked carbs on 70-73 frames in more detail, and with pictures, in this thread:
http://www.xs650.com/threads/xs1b-texas-resurrection.56545/page-3
The above link takes you to page 3 of my "XS1B Texas Ressurection" thread (build is on hiatus, but still in progress). The relevant post is #41, right at the very top of page 3.

To repeat what I previously wrote, you will use the straight 70-73 joints (intake boots). You can also use the later 74 and up angled joints, it will all fit, but then you won't be able to use the stock air boxes, and even if you use pods you are probably going to have side cover interference issues.

 

[DogBunny]

One more thing. I modified my post #2, above in this thread. I initially left out that you also have to remove the top "butterfly" clamp when installing or removing linked carbs on 70-73 frames.

All-in-all, it is a bit tedious taking linked carbs on or off a 70-73 frame. Nothing difficult about it, just a lot of stuff that has to be removed: air boxes or pods, butterfly clamp, gas line, throttle cable, intake manifolds, and don't forget the two big rubber hoses that connect to the engine breather box that are in the way.
If you think you are going to be messing with jetting, this is where you want to replace the Phillips-head screws that hold the float bowls on with Allen-head bolts. As fumbly as it is, it's a lot easier to drop the bowls to change your jetting than it is to remove the whole carb set. Just be cautious not to over-tighten those Allen-heads, if you do make the swap. Very easy to strip out the threads in the carb bodies, which is why I usually recommend against using Allen-heads.

 

[5twins]

You need to remove that top butterfly shaped bracket when fitting a linked carb set in the later frames too. They won't pull back far enough to maneuver them in and out with it in place.

 

[grizld1]

Re. aftermarket carbs, the Keihin FCR flat slide pumper is a work of art (throttle shaft rides on needle bearings, slides ride on nylon rollers, beautiful workmanship) but very expensive, and on top of the high initial price you'd need a custom throttle shaft and bracketry.The Keihin CR round slide is a dedicated 4-stroke carb with cable top slide lift. It's much more economical and easier to work with. If you want good flat slide 4-stroke pumper carbs, Topham Mikuni in Germany offers a 34 mm. pair set up and baselined for the XS650, complete with throttle shaft and brackets, at a very reasonable price: www.mikuni-topham.de . The VM34 is the least expensive viable option; that's the best I can say for it. Lectron? Opinions may differ, but I wouldn't use them on a street bike, and IMO the price is far too high for what you get. There's a long and detailed thread on this page with owner reports on the Keihin CVK34s that were OE on the Kawasaki 500 Ninja. They're newer flat slide vacuum carbs and have produced good results.