Need a proper O-ring size for the fuel mixture screw on a BS38 79' Carb

[Rip_ride]

Anybody had to order new O-rings for their BS38 carb mixture screws and had them fit? Mikes doesn't guarantee sizing and mine were wrong. They sent me rings with an outer diameter of 6.5 mm and it's too big. I'm thinking I need more like 6mm. Anyone have specs for the original size and maybe a suggestion as to where to get them?

Thanks,

 

[5twins]

I'm not sure what the original size was, probably something odd, but a 1.5mm thick x 3mm I.D. will work. That would give you the 6mm O.D. you're looking for. It's a bit snug going in the hole so you need to screw it in "wet" with WD40, but they do work, I've used them. I got them in quantity from McMaster Carr.

http://www.mcmaster.com/#metric-o-rings/=g5fxbl

 

[bluebikerblan]

Here's a chart of the sizes of the various o-rings and butterfly throttle shaft seals
for the Mikuni CV carbs. Data largely supplied by 5twins of this site. Refer to the o-ring
dimensions in drawings below.
~~~~~~~~~~~~~~~~~~~~~~~

Mikuni CV carbs BS Series O-rings Seals XS 650

___________ Carburetor wd. x ID x OD Location

1970-1976 BS 38 I 1.0 mm x 4.0 mm needle jet nozzle

1977-1979 BS 38 II 1.5 mm x 3.0 mm air mix screw
1.0 mm x 4.0 mm needle jet nozzle

1980-1983 BS 34 III 1.5 mm x 7.5 mm float needle
1.0 mm x 3.0 mm air mix screw
1.6 mm x 4.1 mm float bowl drain screw

1970-1983 38 & 34 mm 1/16" x 5/16" x 7/16" butterfly seal quad *
1.9mm x 7.8 mm x 11.6 mm " " round
OEM #256-14997-00-00 " " flange **
Mikes XS #48-5021 " " " **

* AS568A-011
** Not available in Viton...all others can be sourced in either Buna N or Viton
McMaster-Carr

 

[Rip_ride]

This is great guys! Thanks and ride on!

 

[haxto]

I just did the O-ring replacement on my butterfly/throttle shafts. Wish I could say how easy it went but I AM getting must faster at pulling these carbs off and on!
What I came up with to support the shafts while staking the screws might be helpful to someone.
Find a short Phillips bit that fits well into your screw heads and measure how much the bit sticks out. (1st photo)
Get a piece of wood and drill a hole to that depth you just measured. (2nd pic)
Place the Phillips bit in the hole. 3rd pic
Carefully insert the bit into the screw head and place it on a firm surface to stake your screws. 4th pic
Leaving the hole just a little shallower than the measurement will help you be certain that its in place.

 

[5twins]

Thanks for that idea. Looks like another 650 special tool is in my future.

 

[xs650dreamer]

Does anyone have some spare throttle shaft orings ?

 

[5twins]

The throttle butterfly shaft doesn't use an o-ring, it uses an actual seal. They are still in use on Yamaha CV carbs almost up to the present day so are still available from a dealer. This is where most of us get them now because the quality of the ones MikesXS sells can be hit and miss.

http://www.boats.net/parts/detail/yamaha/Y-256-14997-00-00.html

 

[xs650dreamer]

5twins if you look at the bottom of post number three it list the oem seal and the oring replacement dimensions. I think some viton or buna n orings would be real happy there and last a good while. And would not shrink or harden with today's fuels

 

[5twins]

If you do try an o-ring, I suggest putting them in the left side only to test them. You can access that without having to remove the butterfly plates. Some have reported that a regular o-ring and/or the quad style don't last very long. I'm the one who found the quad style several years back. At that time, Mike's was selling the repro seals for like $5 each. I wanted a cheaper solution. Of course, then I discovered you could still get originals for like $2 back then (now they're about $3). That's all I use now. I do have the quad style o-rings in one of my spare carb sets but I haven't run them for very long (they're spares/back-ups) so I can't attest to the longevity of the substitute seal (quad o-ring).