BS34 - 79, or later?


XS400 New Member
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Hello everyone.

First post! I've done a bit of searching, but have a couple of questions about some carbs I've just bought - as I have a bit of time to wait for them to arrive, and was hoping to get some other bits ordered whilst waiting.

I've got a '79 250 frame, which I'm putting a '81 (14V - SJ) 400 engine in... The 400 engine has come with some BS32's, as I measured it against the carbs off my 250 engine (Non runner - missing lots of bits). Which is very boring...

So, I've ordered the carbs in the photos bellow - which have been sold as '79 BS34's from a 400. I've seen some other pictures, and these look like the '81 onwards carbs.. Could any of you geniuses work this out from the photos? From searching I think that any SOHC BS34 will work, but the parts between varying models aren't compatible.

I'd like to order some jets. Am I correct in purchasing:
Main Jets: Large Round Type
Pilot Jet: BS30/96 Type

Again - sorry, did a search - but have found responses for both large and small round type, although it does lean more towards LRT.

For the slide / diaphragm, I'm thinking I need to wait until I know what year the carb is, as they're different depending on the year?
I was thinking of getting these:

And any ideas if this gasket will fit?

The manual for a '81 SJ:
Jet (Slide) Needle = 5GZ9
Needle Jet - Y2

But I can find neither of these for sale. Should my plan be to clean up what is supplied, and re-use them?
I'm guessing that if they aren't the same carbs from the 14V engine anyway, the slide type won't be the same anyway?

Also - seller of the below carbs said the slides are stuck, any tips on anything to use to help free these that doesn't damage the rubber membrane?




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The engine is indeed painted black.. Something on DVLA mentioned the bike was on an '80 plate, not really sure how... And you're right, the manual says Oct '81 - 82.

So through a pure fluke, I might've ended up with a matching set!
Those still have the factory pilot mix screw caps over them. You will need to remove them in order to tune the bike correctly. Stick to genuine mikuni jets as aftermarket ones are junk because of manufacturing issues. Real mikuni needles and needle jets are very hard to find new. Unless yours are very damaged they should work fine. All my bikes run used ones without any issues.
I'll look around for genuine, but you're right - they're hard to find.

*Hopefully* everything behaves, comes apart as it should!

I have 2 x BS32's, do you know of any resources I can use to see if any parts are shared?
Update: Carbs came apart well, and the stuck slides were just some gunk. A load of carb cleaner and a nice afternoon scrubbing and they've come up pretty well.

Pilot jet changed to 45, mains to 147.50. Started on the button once fitted, and running fairly well. It's not on the road yet, so I guess time will tell!

Caps a bastard to get out, so I just drilled them very carefully until a small screwdriver would fit in to allow adjustment.

Thanks for your help.