BS 38 Carb Diaphragm Replacements


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Brigham City, Utah
Please note that a new diaphragm replacement part is being made by JBM Industries

These have been designed and produced by the same gent, Donald James that is supplying the new design carburetor boots (previously posted).

I've just ordered two sets and will check them out in an effort to replace torn diaphragms on two sets of BS 38mm CV (type II) carbs from 78' 79' Specials.

To the best of my knowledge there has NOT BEEN either a replacement slide/diaphragm assembly available or a way of replacing just the diaphragm (enabling the use of the old slide) on this type II design version which uses the E clip method of attaching the needle jet.

If this proves out a lot of good old BS 38's could be brought out of retirement or kept in service!

Hey carb gurus.....have you checked this out? :thumbsup: I'm also advised that a similar design for BS 34's is in the works. :bike:

Till now I have not been able to find a way of repairing or replacing diaphragms for 78' 79' BS 38 CV's and I've got a few that are down for only this reason. Best, Blue
Aw shucks RG...."a running bike"....not sure I can do that right now, I've only got three
to choose from. Happy Spring! Blue

(My 78' SE "Ichi Belle" will tell us!......I'll try changing nothing but the diaphragms)
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Hi Waldo....Thanks for the input. I saw your testimonial on eBay along with some others too. I ordered my replacement diaphragms and carb joint boots Sunday, they got here Wednesday, I put one set on my best bike yesterday. What a snap to install on the slides! I couldn't believe it. With the carbs off the bike....two tools: my JIS (#2 Phillips type) Vessel screwdriver & a small very sharp new set of dike cutters.

30 minutes being super careful not to knick, notch or scrape the slide while cutting off the two Nylon rings which clamp the old diaphragm to the slide on the first carb. 10 minutes on the second.

Piece of cake putting on the new piece, much like putting a teeny tire on a tiny rim....perfect fit, rolled, operated and bench tested superbly when re-assembled.

Couldn't fire Belle up yet, her tank's off getting Red-Kote lined (which is now Blue btw)

JBM carb boot joints appear very beefy, heavy & strong. Mounted. Again, no operation yet even though spring has sprung in the Rockies! I'm now officially late getting my best bike "Belle" (Blue of course) a 78' SE back on the road. Two others with tanks off, not in the game yet either. :banghead:

Night. Bedtime, Blue
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One size fit's all BS Series 38's! Same DIY part and method. You call they haul that's all! No :bs: Blue

Update Both carb types I & II(BS 38's from 77'D and 78'SE) up and running (static tests) no discernible differences so far. Looking good indeed!

PS Oldskoolcarbs, Rock Hill, SC does a fabulous carb rebuilding job!
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Hi Blue!! Digging in your old post here -- as per this page,

Does it ship as a diaphragm only, and I can mount it to my original metal slider that simply?

Turns out at least the right carb on my '72 (bs38 carb) has this slightly torn diaphragm. Resisting the temptation to just super-glue it as I don't think that'll last! (not sure if the prev owner ignored it or glued it).

Thank you! hope this message finds you,
Yes you mount the JB diaphragms on the stock piston.

:cheers: Hey blan sorry to stomp on your thread :D
I'm in the process of replacing the rubber membranes on my BS38 CV Carb Diaphragms that I bought from MikesXS with the ones from JBM Industries. So, in other words, I'm not refurbishing stock throttle valves.

Is is safe to assume that the throttle valve bodies (metal cylinders) from MikesXS and OEM Stock are identical in physical dimensions such that the fit of the JBMI diaphragms is the same? I sure as hell hope so.

I've completed one (per YouTube video linked on JBM web page) and while it was easy enough seems to leave quite a gap (see last image) under the mounted diaphragm. It makes me wonder why the lower plastic ring couldn't be left on.

Images 1 and 2 are carb diaphragms (from MikesXS) prior to removal of the top and bottom rings and rubber membrane. Images 4 and 5 show the newly installed diaphragmn from JBM Industries.


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I don't think you're supposed to remove the top ring, just the bottom one.
Here is Don's reply:

Hi _____ ,

It looks like you have them on OK. I do not know if our parts fit on the aftermarket slides, but your photo looks like it is OK. You can use Crazy glue or Super Glue, or put a snap ring on as it looks like you have the space to do it.

Don James
JBM Industries
3761 Morley Drive
Kent, OHIO 44240

I'm going to pop on a pair of big snap rings (although it's probably not absolutely necessary) and call it good.
How about o-rings?
Excellent idea, 2Many!

I had some 25mm ID (~3.5 mm W.T.) O-Rings on hand that seem to fit nicely.

Not sure of composition, but hope that they are NBR (Buna-N Nitrile) so as to be fuel resistant.


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Great! Now you got me wondering, no carb in front of me and clueless, what arrests the downward travel of the slide? The slide bottom, or that now missing plastic ring?
The throttle valve bottoms out in the carb body leaving plenty of slack in the rubber membrane. This is what I observed with or w/o the jet needle in place, but I guess the jet needle landing in the main nozzle (aka emulsion tube, needle jet) occurs more or less simultaneously.
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I read someplace these are just rubber. The originals at least on the bs34 are cloth covered with rubber, which is typical way of making things like this that need to last awhile.