BS38 slide diaphragm

[The Phantom]

Hello again. Feel like I’m being a pain. Oh well, I have one diaphragm that seems to leak down slowly. The other one is solid. Can I get just the rubber or do I have to buy the whole slide? Should I get 2? Who should I trust for good quality? Thanks
I started the thread on this bike now.
PHANTOM RESURRECTION 78 SPECIAL. Thanks Jim.

 

[Jim]

I have one diaphragm that seems to leak down slowly.

Define "slowly."
If it'll go about 8-10 seconds or so before it bottoms out, it's OK.

 

[Jim]

I'll make a suggestion... Start a thread on your bike. Call it a resto, mod... bringing back a survivor.... whatever you want, then ask all your questions there. You'll have an easy to follow journal of what you did that's easy to reference back to... and makes it so we can follow along on your progress.
Scattered across multiple threads like this one is the hard way to go. Make it easy on yourself.

 

[jpdevol]

So the standard "rule of thumb" for a diaphragm test is to block the oval intake port in the bell and it should take ~14 seconds for the slide to drop from full open to closed. Replacement diaphragms are available from JBM http://jbmindustries.com/Yamaha650.html - if I'm doing one, I'm doing both. The procedure is a bit fiddly, sometimes I'll opt to buy the whole slide assy. with the diaphragm in-place https://www.mikesxs.net/yamaha-xs65...70-77-bs38-oem-256-14940-00-256-14940-01.html

 

[Jim]

JP, where'd you find the 14sec number? I've searched high and low.... never found a number...

 

[jpdevol]

It's here somewhere....maybe the "carb guide"It 's valid AFAIK & IME

 

[5twins]

It's possible you're not getting a good seal between the carb body, top, and the lip around the outside edge of the diaphragm. First, I'd pop the top off that carb and make sure the outer lip of the diaphragm is seated properly in the groove provided in the carb body. I've also taken to smearing the entire diaphragm with red rubber grease, especially the outer lip. I think this helps it seal better .....

 

[650Skull]

Wet your thumb and make sure you have a good seal................Really easy to have a leak around the thumb, when doing the drop test

 

[The Phantom]

I'll make a suggestion... Start a thread on your bike. Call it a resto, mod... bringing back a survivor.... whatever you want, then ask all your questions there. You'll have an easy to follow journal of what you did that's easy to reference back to... and makes it so we can follow along on your progress.
Scattered across multiple threads like this one is the hard way to go. Make it easy on yourself.

Will do. I appreciate it much

 

[The Phantom]

Define "slowly."
If it'll go about 8-10 seconds or so before it bottoms out, it's OK.

Thanks, by your description it’s ok

 

[Jim]

Thanks, by your description it’s ok

Well, I'll defer to JP's 14sec comment, but....
I've got a test engine that's running BS-34's. One of the slides drops in about 6-8 seconds. It runs just fine like that.

 

[The Phantom]

Thanks, by your description it’s ok

Went back to check, 3 seconds, not slow enough. I’m going to start my resurrection thread as advised. Thanks

 

[toglhot]

Yep, JBM industries sell the rubbers. You'll have to remove the plastic ring on top of the old rubbers so you can replace them. Apparently the JBM rubbers do tend to turn on the slides, but a dab of superglue fixes that problem.

That's the reason I did a VM carb conversion: JBM weren't posting to Oz at the time and a complete slide, with rubber, was $120 Aust, thats $240 for both, plus my time fixing the damn things. I don't particularly like vacuum carbs, so I bought a couple of VMs and made new manifolds for them. I'd much rather spend my time turning up something on the lathe than rooting around with rubber diaphragms.

As JBM are resident in the US (Ithink) you shouldn't have any problems obtaining them, and the rubbers are as cheap as.

 

[toglhot]

Incidentally, before doing the VM carb swap I did try fixing the diaphragms. First off the thick ring of rubber had been squished beyond redemption over 45 years, so I cut a gasket to sit on top. That solved that problem, but then the diaphragms started developing pin holes in them, so I plugged them with Sikaflex. That worked, but after a while the pin holes became so numerous I quit.
Here's a video,on softening hardened rubber:

Apparently soaking in wintergreen oil also works. Soaking in silicon spray is also supposed to work, don't bother, it doesn't

 

[BowlingSS]

So the standard "rule of thumb" for a diaphragm test is to block the oval intake port in the bell and it should take ~14 seconds for the slide to drop from full open to closed. Replacement diaphragms are available from JBM http://jbmindustries.com/Yamaha650.html - if I'm doing one, I'm doing both. The procedure is a bit fiddly, sometimes I'll opt to buy the whole slide assy. with the diaphragm in-place https://www.mikesxs.net/yamaha-xs65...70-77-bs38-oem-256-14940-00-256-14940-01.html

Big difference in price but if you need the slide. Could buy a used slide and install JBM’s. I have installed a few with good results.