BS 38 Carb Diaphragm Replacements

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.

That's right, on my 38's they're rubber coated cloth. On old H*ndas same story. Fixed a couple vacuum diaphrams (dashpots) on my cousin's 87 Civic using that Napa Plastidip spray, the cloth base gave it good footing.

Got a couple of JB diaphrams waiting to go on, they look, feel, and are reported to not have cloth inner, uses new/different rubber material, I guess doesn't need the cloth...
FYI on the slide groove dimension, JBM reports that the factory BS38 slide groove is 4mm wide.


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For clarification, is the "slide groove" the top of the cylindrical throttle valve that the diaphragm hugs around?

Update: According to Don James of JBM, the "Slide Groove Width" shown in their dimensions chart for BS38CV's is the: "Width of the "tire" and it fits in the groove in the slide after the plastic rings are removed".
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Hi guys. Ive just performed this carb diaphram replacement as well as new intake manifolds on my 78' 650 'Special' because the bike was running rough & 'popping' on overrun. The bike fired up instantly & is running fine except when i rev the bike she revs clean (no popping) & smooth but is slow to return to idle. Any ideas?

In reference to posts #13 & #14, I found some BS38CV Carb Diaphragms on eBay. For everything in the attached images, I paid about $16, plus $10 shipping.

A good deal? The rubber membranes are crap and will need to be replaced.

Flashback: When I bought my pair of slides from MikesXS (in 2012), I stupidly threw away my original pair of stock slides. Not realizing at the time that I could have refurbished my slides with the JBM products, I've been kicking myself ever since. This eBay purchase sure ends the self-recrimination. :D

Two of these slides have the jet needles stuck. The needle-fitting plates have plastic sticks (handles?) pointing upwards toward the covers. I've never seen these before. Ideas on safely removing?


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+1 there are several different BS38 slides they need to stay with the body model they are designed for. Sorry but you may not be done kicking yourself for throwing away the originals.
The top two slides in your pic are '78-'79 type. That plastic "stick" is the needle retainer and is held in place by a snap ring .....


The bottom two slides may be what you need, retaining the needle with those plastic discs above them. Can't tell what the needles are. You'll need to clean them off and read the numbers on the sides. Your '76 carbs use a 4M1.
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Yep, at least one pair is of use to me. Still feel good about the buy since I've seen XS650 carb slides priced anywhere from $50 to $100 EACH not to mention the needle retaining plates listing anywhere from $8 to $24 each on eBay. I've measured the slides from MikesXS and the eBay ones (that fit my C) and they are identical in dimensions.

As for the two slides from the '78-'79' (thanks for the diagram 5twins!) it was obviously the retaining clip that kept me (initially) from pulling out the plate and other parts. To be specific, what I removed were actually two internal retaining (snap) rings as opposed to the e-clips or E-Style retainers shown in the parts diagram.

Needles include: 2 x 4M1's, a 4N8 and a 503 502 (?) (all Mikuni). There was a 5th needle, but I bent the shit out of it.

On a somber note, this evening my wife's fucking Doberman Pinscher ate one of my new JBM diaphragms that I had removed from the slide to compare the sizes. I had left the room to get my phone to take the attached images. :cussing:


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The 4N8 is from a '74-'75 carb. The 502's are stock for the '78-'79 set. The first number in a Mikuni needle part number designates its series. The series run in 10mm increments. A "4" series like that 4N8 or your 4M1's will be between 40 and 50mm long. The "5" series like those 5O2's will be between 50 and 60mm long. A "6" series would be between 60 and 70mm long, etc.
Was the needle retention system in the '78 - '79 and all those little parts seen as an improvement from the much simpler system (i.e., plastic plate) used in earlier years? I'm wondering what issue they were trying to fix or improve upon.
Yes, it was an improvement because the needle was now spring loaded. This kept it more stable and made for more precise metering. If you look at the later BS34s, you'll see their needles are spring loaded as well. I think these spring loaded needles were one of the steps taken to help these carbs meet E.P.A. specs. I think they do make for a smoother operating carb.

If you grasp the needle where it hangs out of the bottom of the slide on your '76 slide, you'll notice it has some up and down play, maybe about 1/2mm. The needle is free to move up and down that little bit during operation which makes for less precise metering.
Despite the benefits of this retention system, I assume that is ill-advised (in concert with Gary's earlier comment) to use these slides in an earlier carb so as to take advantage of the smoother operating benefits?

I wonder whether it's been tried (successfully)? I would think that one would have to use a different needle, but that's a WAG on my part.
Yes, you would need a different needle most likely. The 5O2 needle was paired with Z-2 needle jets. The 4M1's and 4N8's were paired with Z-8 and Z-6 needle jets respectively. The tapers are most likely different. Even though the 5O2 needle is longer, to make room in that newer style slide for the little spring, it was mounted higher up in there. If you compare the amount of needle that hangs out the bottom of the slide, you'll find that your 4M1's are about the same as the 5O2.
Someone please clarify that I have may needle reassembled correctly. Based on 5twins post above (#30) the small spring is installed under the metal washer before inserting the needle into the slide body. The reason I ask is because I was certain that during disassembly I thought it was place above the thick plastic spacer at the top of the needle.


The location of that little spring would depend on the carb set you're using. The diagram above pertains to the '78-'79 BS38s only. If you're using any of the BS34s, the little spring is mounted on top of the needle clip and spacer washers .....

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