BS38 Carb - considering rebuilding

Niels B

Niels B

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What’s your guys take on these ? Have a bunch of new and used parts for these carbs, and was thinking about refurbishing them. They are most likely from a ‘74 TXA of ‘75XSB, based on the jets 127.5 & 45. However after cleaning and starting on the disassembly I learned that the left is heavily corroded where the float pin connect the float. Float still moves but… see picture.
On the right it’s missing a pipe - not sure if the name and if it can be replaced see pic.
Should I just save what can be saved and trash the rest or do you think there is a way forward?
 

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Niels B said:
What’s your guys take on these ? Have a bunch of new and used parts for these carbs, and was thinking about refurbishing them. They are most likely from a ‘74 TXA of ‘75XSB, based on the jets 127.5 & 45. However after cleaning and starting on the disassembly I learned that the left is heavily corroded where the float pin connect the float. Float still moves but… see picture.
On the right it’s missing a pipe - not sure if the name and if it can be replaced see pic.
Should I just save what can be saved and trash the rest or do you think there is a way forward?
Click to expand...
If you what a set, they're easy enough to come by. I would shitcan those.
 
That’s my feeling too, the right one may be salvageable by using what it’s missing from the right - then take whatever parts that’s ok and trash the rest. I tested the floats and they are surprisingly not leaking :)
I don’t mind a challenge but I don’t like the look of the float holder :)
 
Bead blast them before doing anything, there may be sufficient metal left. Whatever you do, don't try and clean them up by hand using a wire brush or whatever. As long as the pin hole is ok, that's all you need, there is absolutely no force on them from the floats, so it really doesn't matter if they are eaten away a little by oxidisation. The oxidising looks like it has built up on them, so, they may be alright.
 
Personally, I wouldn't bother with an unlinked set, unless they were needed for a resto. That left carb does look pretty bad. Besides the float pivot posts, the area around the float needle seat looks pretty bad, and that might not allow it to seal into the carb body. I think the right carb is fine. I don't think it's missing that tube, Yamaha left it off because there is no choke on that carb. The right carb was fed it's rich starting mix through a rubber hose from the left carb, which did have a choke.

If you do decide to go forward with this and seek out a replacement left carb, there was a big change between the '73 and older and the '74 and newer carbs. The floor in the main bore was raised on the '73 and older carbs .....

EarlyRaisedFloor.jpg


The '74 and later carbs had a lower floor which made for a rounder intake tract .....

Late Carb Throat.jpg
 
Thanks for the Education on The carb 5twins, I knew of a few changes but hadn’t come over this info before. I have another set I did restore a while back - now I’ll have to take a closer look on the year model - reason I thought the right is missing the tube is that the other set I have did have a tube on both carbs.
I’ll update here what I find out - but was in doubt about the left for sure - the right look kinda ok but as mentioned thought I was missing that pipe
 
5twins said:
Personally, I wouldn't bother with an unlinked set, unless they were needed for a resto. That left carb does look pretty bad. Besides the float pivot posts, the area around the float needle seat looks pretty bad, and that might not allow it to seal into the carb body. I think the right carb is fine. I don't think it's missing that tube, Yamaha left it off because there is no choke on that carb. The right carb was fed it's rich starting mix through a rubber hose from the left carb, which did have a choke.

If you do decide to go forward with this and seek out a replacement left carb, there was a big change between the '73 and older and the '74 and newer carbs. The floor in the main bore was raised on the '73 and older carbs .....

View attachment 260014

The '74 and later carbs had a lower floor which made for a rounder intake tract .....

View attachment 260015
Click to expand...
So looking at these they may be ‘74 or newer ? The black and the raw have the same bore - don’t see any raised floor. The left is the crabby one the black is part of a restored set I did, the were jetted 130 / 42.5 so when refurbishing I used new set of jet same size. That made me think they might have been ‘73 or earlier that’s the jet shown for those years.
 

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Well that sealed it - both sets are ‘75, any good reason why they would have been jetted as for ‘74 or earlier ? I was of the impression the jet followed the carbs - not the bike year they sits on - as I’m sure they would all work on different model year as long as they are BS 38’s or by use of adaptor if it’s BS34’s
 
74-75 jetted same.
As to different jets used, the mind of a PO is often inscrutable. One guess is Many, most, all carb kits come with the wrong size, aftermarket jets. The odds of it running well after a bumbler installs a random carb kit are slim.
IMO Correct MIKUNI jets with the little square in a square logo are the only way to fly.
650carbspecsreducedsizeey7.png
 
Jim said:
Jus' throwing this out there.....
Damaged and/or eaten away float towers can be repaired without too much difficulty.


View attachment 260040
Click to expand...
Nice job there - looks like there was a little more material to work with though :). I’ll vapor blast and ultra sonic clean this body and see what’s left - then take it from there :)
 
gggGary said:
74-75 jetted same.
As to different jets used, the mind of a PO is often inscrutable. One guess is Many, most, all carb kits come with the wrong size, aftermarket jets. The odds of it running well after a bumbler installs a random carb kit are slim.
IMO Correct MIKUNI jets with the little square in a square logo are the only way to fly.
View attachment 260033
Click to expand...
When I refurbished my first BS carb I followed suggestions from this forum to use Mikuni jets - the once in these old beaten up carbs are original mikunis too - but they are not salvageable
 
So I guess I had to much time on my hands and needed to get busy. As I sorted through the piles of new & old Yamaha parts found a couple of new sets of Mikuni jets for the ‘75 carbs. I ended up giving it a shot of rebuild, lots of vapor honing with glass beats and a couple of ultra sonic baths @ 75°C. Some chemical and gas prove JB weld and then build up. I’ll test on an engine I have on the stand that I plan to hook up to fuel and power at some point, but I’m pretty confident they will work. Waiting on a throttle lever for the left side carb ( found one on eBay - a $10 rib off :) Also had to be inventive on the throttle bracket, they are a little lumpy but will work fine.
 

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Cork float bowl gaskets? The Bings on my old BMWs use them but I've never seen them for these Yamahas. You cut them yourself I take it?
 

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