JBM Industries BS38 Diaphragms - Fail?

[TwoManyXS1Bs]

Started getting off-idle stumbling, hesitation, on left XS1B BS38 carb. Pulled carbs, left carb failed the diaphragm drop test. Plugged starter circuit, still fails. Found the JBM diaphragm was no longer a snug fit on the slide. Easily rotated, easily removed.

It appears that the diaphragm has swelled slightly, providing a loose, leaky fit. Even the outer ring is larger, almost too big to fit in the carb body groove.

I installed these about 1.5 years ago. Using only hot soapy water, the diaphragms were quite a snug fit on the slide necks. And, after drying, were rock solid locked in place.

 

[TwoManyXS1Bs]

Pulled the right carb diaphragm, same thing. Loose, swelled diaphragm. Just hadn't started leaking yet.

Thoroughly cleaned the slides and diaphragms using hot soapy (Dawn) water. After drying, applied a thin smear of super-glue to the neck of the slides, reinstalled the diaphragms.

 

[TwoManyXS1Bs]

Both carbs now pass the leakdown, drop test.

A perfectionist's addendum to slide/diaphragm alignment:

The JBM instructions state that the slide is to be installed such that the slide's bottom vent hole be toward the engine side of the carb. But, the vent hole isn't necessarily in alignment with the slide's cutaway.

I recommend taking this a bit further, by aligning the slide's cutaway ramp perpendicular to the throat, so that it fully covers the raised floor of the early carbs. To get this alignment, simply raise the slide halfway, and check for equidistant positions of the slide's cutaway corners and the carb's slide bore edge, as in these pics:

 

[xjwmx]

I read reports that sometimes they needed to be glued when installed to keep the piston from turning. I think they and the boots are passe now, thankfully.

 

[TwoManyXS1Bs]

Update: Found an old thread addressing this issue.

http://www.xs650.com/threads/jbm-diaphragms.17057/

 

[650Skull]

A couple of other threads i had bookmarked........

This thread has an interesting observation by bluebikerblan, Post #18, also reiterated by JBM industries
http://www.xs650.com/threads/carb-slide-rotating.11829/

http://www.xs650.com/threads/jbm-diaphrams-and-boots.1994/