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.
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!
I have found the problem. Actually two problems. One I caused.
The carbs are still racked but on a bench. I was trying to figure out why one worked right but not the other. I found on the good carb when holding the slide up under vacuum if I raised the choke lever the slide fell. Aha, I have an area to look. Found the choke plunger on the bad carb wasn't fully seating. Polished the varnish and it worked much better.
There was a bigger problem. When I first took the carbs apart one of the bowl screws was stripped. I found a slightly larger screw and tapped the hole and problem solved. But I had accidentally nicked the mating surface between the carb body and choke cover. It's such a tiny area and an awkward spot to work on but it's fixable. Lesson learned.
