I've encountered BS 38's with slide's that have rotated a number of times and found this problem not to be caused by or associated with the diaphragm...Whether the OEM type swaged in between the two nylon rings or the JBM DIY one piece molded in snap fit integral rubber washer design.
Rather, I've found that the rotation of the slide seems to be occurring when the slide surface comes in contact with some desposit, debris or damage on one side or the bore other as it travels up and down. This gunk deposit can be quite hard and stuck to the slide surface itself or more likely....the inner surface of the slide cylinder bore in which the slide travels up and down. Usually it's intake air carried deposits on the aluminum cylinder wall near the top. I suspect this gunk to most likely be from crankcase gas vented through the breather tubes to the air boxes and thence into the top oblong air intake port in the carb throat bell and then to the CV diaphragm chamber, slide and bore.
PREFERRED REMEDY
Locate and remove the interference (that is causing the slide to rotate) by gentle cleaning preferentially to trying rotation restriction by gluing the diaphragm to the slide
Usually fairly easy to detect and correct by careful removal cleaning and deposit elimination.....
1. Remove the 4 screws of the cylinder top cap and remove cap & slide spring
2. Carefully loosen the diaphragm from it's perimeter groove in the slide cylinder body and remove it together with the slide assembly. Examine the slide surface for dirt, deposits or damage. Clean with a soft clean cloth (tee shirt, cheese cloth etc.) and WD 40.
3. Carefully examine the now exposed entire polished aluminum surface of the cylinder bore by both sight and feel. I've usually detected a ridge like bonded piece of glue-like varnish near the top edge which can usually be softened and wiped out with a soft cloth and WD40. I've also used a small dab of chrome polish like Simichrome to clean off the deposit spots and also to carefully & gently re-polish the smooth surface of the bore aflter cleaning. No abrasives or metal removal! Sometimes feel detects the crud or deposit spots for cleaning better than sight.
Gently clean and re-assemble, test for smoothness of slide operation. The "rotating slide problem" is usually solved. A very slight amount of cyano acrylate (super-glue) can be applied between the rubber tire of the diaphragm slide metal groove to help prevent rotation if you wish but; don't get any on either the slide surface or cylinder bore!
Sorry No Pictures! Hope I've made it clear ask away if not. I happened to be doing some carb sets now and fixed two such gunk spot infected bores with rotated slides today. Of the 10 or so used BS 38 carb sets I've purchased and tried to set right, I guess I've encountered 6 or 7 individual carbs with the problem. So far, all have been corrected by the process described. Moreover, I prefer not to bond the diaphragm to the slide.
BTW....You can probably guess how I feel about the venting of dirty oil laden crankcase breather tube gases into the air filter boxes and therefore into the carbs and CV chambers but; that's an EPA emissions matter as you might guess...ahem. Blue
