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JBM Industries BS38 Diaphragms - Fail?

Discussion in 'The Garage' started by TwoManyXS1Bs, Mar 18, 2018.

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  1. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    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.

    71XS1B-JBM-DiaphragmsFail01.jpg

    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.
     
    Last edited: Mar 19, 2018
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  2. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    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.

    71XS1B-JBM-DiaphragmsFail02.jpg
     
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  3. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    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.
    71XS1B-Carbs-Slides02a.jpg

    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:

    71XS1B-JBM-DiaphragmsFail04.jpg

    71XS1B-JBM-DiaphragmsFail05.jpg
     
    Last edited: Mar 19, 2018
  4. xjwmx

    xjwmx XS650 Guru Top Contributor

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    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.
     
  5. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

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  6. 650Skull

    650Skull Dinosaur Lives Top Contributor

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