1977 Yamaha XS650. BS38 Carb tuning/specs

[rafanomenon]

Hey all,

I just picked up a 77 xs650 bobber project. As far as I know, the carbs that came on this bike from the factory were Mikuni BS38s. The bike didn't come with headers or air filters, so I went ahead and bought some Uni foam filters, and some stock headers that I plan to run wide open.

I've done a little looking around, and may have found a baseline:

Idle/Pilot jets: 1 up from stock
Main jets: 2 up from stock
Needle clip: 1 notch down from middle

I was going to buy some VM34s, but I want to keep this build fairly cheap, and heard that 77-78 were some of the last years with the BS38 carbs that allowed for easier tuning.

I was wondering if anyone out there was running a similar setup, and if so, what your carb specs are?

Thanks in advance! Can't wait to dial this baby in...

 

[Big R]

Hey Raf, i have a Suzuki 650 twin with stock BS36SS carbs. I know they are different, but i can open them up and see what's in them if you want.

 

[teazer]

Just zip on over to mikesxs.com They have replacement needles and jets etc. For your changes, one up on pilots and 2 up on mains is a good place to start and raise the needle 1 or 2 clip positions.

There were several very different incarnations of the BS38 and jets and needles are different so be sure of what's in yours first. Choose the wrong ones and they will not work.

 

[crazypj]

You have to go here and sign up.
After you sign up you can look at carb tuning guides
It's not the definitive article on XS carb tuning but it's about a close as you will get
http://xs650temp.proboards.com/index.cgi?

 

[ryanglade]

I agree with PJ. I found a pretty awesome guide on the carbs on the xs650 that goes into the different models and the adjustments the should and can be made. I printed it out and referenced it when rebuilding mine and will a lot when I go to dial them in. You will probably find the best guides on that site over other sources.

 

[rafanomenon]

Signed up and got the guide. Looks like I've found the XS650 carb bible

Pipes and pods are shipping this week. I hope to have the carbs pulled, cleaned, rejetted and tuned by this weekend, depending on the crappy September Seattle weather...