help identifying 34 and 38 mm carbs

the-oz-slider

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hey ya'all,
I have just finished getting my Xs1-B on the road and I continue to be haunted by carb issues .......
I have used my XS1 as a reference point, but am now starting to question whether the carbs on it are correct. First question is, how are the carbs measured for size? Ive always though it was the diameter of the intake hole on the engine side of the carb....am I wright or wrong?
The book says 34mm is the correct size for both bikes, but the diameter of the throat on the engine side is 38mm +/-.
Also, how do you tell what is the correct vacuum slide? they arent numbered. I know we had incorrect needles in the carb, because the bike wouldnt idle under 2500 rpm, and once I put the correct needles in it was fine. Well the correct needles for the particular slides we had in, but on reflection I suspect that I have the wrong slides/needles and thats part of the problem.
Currently, the carbs on the bike have 120 throttle valves, but the book says 125 is correct. I do have a very nice set of carbs with 125 throttle valves [and original SOLEX lids], but once again, the throat size measures up at 38mm+/-. I cant find any reference to any model using 120 throttle valves so I have no clue where those carbs came from -well other than from my Pandora's Box of carb parts.......

A bigger part of the problem is I have a box of carbs and parts and the bike didnt have carbs when acquired, so I built a set from the best of everything, and that may be part of the problem also.......
Help please.....
Bike is all-original except for seat, consumables [tyres, cables, chain etc] and touched up sidecovers. Most chrome is perfect- fenders in particular, mufflers are lousy. The tank top has taken on a beautiful creamy/yellow tone from sun exposure. Im looking for better sidecovers and somewhat better mufflers as I dont want to fit new ones.
the oz slider.....
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excellent, thank you. Ive lots of work ahead of me. Its obvious that I have a mixture of parts as the 1-B doesn't have the choke strap as an immediate and obvious deficiency.
 
but my original and correct factory XS1 workshop manual states that the bore size is 34mm, what gives?
 
Yes, typo I'm thinking. Yamaha didn't even use any smaller CV carbs than the BS38s until 1976 when they put some BS34s on the XS360.
 
that be my thinkin'.....ive rebuilt a set that are correct 38's, correct slide, jets and bits and pieces [but no choke parts for the second unit], so Ill need to find a choke activator metal strip and the few pieces for attachment, but I should be good now....
 
And, in this excerpt from the combined manual, the early "raised floor" BS38 is reported to have a venturi size of 30.6mm. The venturi is that zone between the slide base and raised floor, thru which the needle jet and jet needle exist.

Carb-XS1B-02.jpg
 
OK, great news....no make that fabulous news......
I went back to the very beginning, as its a very good place to start....
And I separated out all the various bits and pieces into 2 piles, being '38 early carbs' and 'other'.
Then I matched all the correct parts for a set of early 38's, and cleaned and assembled them ….
And great day in the morning, she started second kick, and idled and revved perfectly wouldn't ya know it.....
But I have one question.
The early 38 bodies I have contain an idle adjustment screw that has a knurled brass hand operated round adjuster piece about 3/4" across and Ive never seen one before. Im wondering if it was an after-market item by any chance. Either way I loved the look of it so I installed it.
 
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