BS38 diaphragm question

I rebuilt the carbs on my 750 twin with parts from Z1. I tested the original diaphragms per the manual, lift the slide then block the vaccum port, slides slowly lowered on each one, so I left them in. Bike ran great, idle nicely. Last week when I started it started idling badly, erratic rpm, then rpm started climbing to the point I had to shut it off. After checking I found the right side carb slide would immediately drop. So I ordered new diaphragms from JBM. Left side works fine. Right side still won't stay up, instantly falls. I was swapped diaphragms and problem persisted. Am I missing something?

Sounds like a vacuum leak.  Most common source are the rubber carb boots bolted to the head.  And don't be fooled by spraying combustible stuff on the boots, because that doesn't always find vacuum leaks.  

Is the diaphragm seated correctly in the carb body and cover sealing against the diaphragm 

Out of curiosity has anyone drilled-out the lift hole on that slide? You could use something like yamabond #5 around the outer edge of the diaphragm where it sits in the carb casting land, just the bottom not the top. Of course you know those JBM replacements Don't have locating tabs so you have to position lift hole towards engine manually. If Both original and replacements exhibit the same behavior within that same carb then check for a warped cap. Have fun!