JBM Industries Diaphragms

[Toolmaker Tim]

Has anyone here tried the JBM Industries carb diaphragms? I was out bid on the set I wanted on fleabay, so I went a googling and this company showed up. They are asking $19.90 per diaphragm. The interesting thing about them is the don't use the plastic rings that are on the tops of the slides. Their website says that you should cut them off, and the replacement diaphrams have a thick center part that replaces the plastic rings and seals tightly around the slide. They call it a 'tire'. The company is in Kent, Ohio. I may have to nominate myself as the first test rabbit if no one here as tried them or has any info on their product.

[jetmechmarty]

I think you might locate the discussion of it at XS650 Garage or try XS650.com. XS650s use BS38 and BS34 carbs, so be sure to get the correct ones. I haven't heard any negative remarks on JBM. I did read a post about replacement without cutting the ring.

JBM Industries

[Tom Andersen]

I have a set of them, I cut the rings and used no glue and they work great. I talked to the owner, nice guy, very helpful. Be patient installing them, they fit very tight.

[Toolmaker Tim]

Tom, did you install them on your xs1100 and which size did you get?

[Tom Andersen]

Tom, did you install them on your xs1100 and which size did you get?

Tim I did install them, they don't have a tab to locate them like the stock ones so just make sure the slides ore positioned correctly. I got the BS34CV (73mm) size.

[Tom Andersen]

I noticed the web site says XS1100 '78-'80 uses BS38CV but I ordered and used BS34CV (73mm), according to my receipt.

[jetmechmarty]

I noticed the web site says XS1100 '78-'80 uses BS38CV

That has to be wrong. I have several XS650 motorcycles. The late XS650 with the BS34 matches what is on my XS1100SG. As a matter of fact, the carbs on my XS650SH look exactly the same, even though they aren't. The BS38 carburetors are larger and will not fit on the BS34 manifolds. I never compared them, but I expect the slides are bigger, too.

[TopCatGr58]

Hey Marty,

The slides for the 80-81 carbs ARE larger diameter than the ones for the 78-79, that's why the aftermarket ones we can get from PNM and Mikes will only fit the later 80-81 series carbs.

So...it does sound right that Tom got the smaller ones for the 34 carbs since the early XS11's are also smaller!

And Tim has the 79, so also smaller slides! Will be looking for that TECH TIP when you get them installed Tim!

T.C.

[jetmechmarty]

TC,

He said JBM called out BS38 for XS1100. Those are BIG. Admittedly, I don't know very much about XS1100 carburetor applications, but that sounds like it's in error.

[cywelchjr]

Yeah, the late model 650's use the same carbs as the late 1100's. They are fitted different, but are the same BS34III's vs the BS34II's of the early models (which the early model XS400's share). IIRC 79 and earlier 650's used BS38 carbs instead of BS34's.

[Toolmaker Tim]

I'm going to order them tomorrow. My daughter is home from college, so she can take pictures and show me how to get them in the computor. I guess I've put off learning how to do that long enough. As far as a tech tip goes, the JBM website is about as clear as it gets when it comes to showing you how it's done. Still it would be nice to have it right here on Chan.11. Truth be told I haven't even brought my running bike home yet. It's still at my Mom's place, but we are trying to sell it since her passing, so I guess it's about time to bring it home.

[Tom Andersen]

The installation is pretty straight forward, again watch the position of the slides, make a mark where you can see it from the top before you remove the old diaphragms. Cutting the plastic off wasn't to bad, I just kept thinking, I hope this works, it did. Again also, go slow fitting the new diaphragms, they're tight.

[cywelchjr]

Not really a bit deal on the position marking. The hole that goes through for the vacuum to come up through goes fully to the engine side. Just make sure it's 180deg from the intake side of the carb and it will work fine. It's all about getting that bleed hole in the correct position just behind the venturi. If you get it off even and little bit the bike will stumble really bad.

[Toolmaker Tim]

On Their Way

Talked to the owner of JBM today. Nice guy. It seems he does his molding in the basement of his house, if I understood him correctly. With my lousy hearing I might have. If he does do it in his basement (the molding), he must have a very understanding wife. Of course it probably isn't injection molding like I am involved with. It's is probably some sort of thermoset molding.

As far as the bleed hole goes, judging by my somewhat trained eyeball, even on stock, unaltered slides, the hole isn't exactly behind the needle. The slides I have seem to be a couple of degrees to the left of the needle, that is looking down into the slide. I wonder if the positioning of the bleeder hole could be a factor in tuning the carbs. With these diaphragms it would be easy to determine if it is a factor.

[cywelchjr]

Talked to the owner of JBM today. Nice guy. It seems he does his molding in the basement of his house, if I understood him correctly. With my lousy hearing I might have. If he does do it in his basement (the molding), he must have a very understanding wife. Of course it probably isn't injection molding like I am involved with. It's is probably some sort of thermoset molding.

As far as the bleed hole goes, judging by my somewhat trained eyeball, even on stock, unaltered slides, the hole isn't exactly behind the needle. The slides I have seem to be a couple of degrees to the left of the needle, that is looking down into the slide. I wonder if the positioning of the bleeder hole could be a factor in tuning the carbs. With these diaphragms it would be easy to determine if it is a factor.

If they were put on by hand back in the day, that could also be within the margin of error for positioning of the bleed hole. When I got mine wrong on my 400, I had it like 10deg or so off, and it stumbled badly, but I'd think a couple of degrees wouldn't have much effect, being that until a certain distance from the sweet spot was reached, the same venturi affect would take place on that hole causing the same action.