I've never used Shindy, is that an American brand? I manage to get hold of Moose products off eBay and from some U.K. Dealers.just buy a carb rebuild kit !!..lol. shindy or moose brands are good, I've used both in the past, but I favor shindy over moose.
I've used 3 moose valves and used the 250 for 5 to 10 mins, parked it up, tried it again a few days later and happy days I'd cured the issue. 3 weeks later.... start it up leave it ticking over for a few minutes... fuel everywhere! I've tried 3 of these now and 1 other brand, it may have been all balls I can't remember now.
As you all know, I've had this carb apart and cleaned that many times i could do it blindfolded now.its definitely clean, I have no doubts at all. All I can think is that it's the quality of the valve or the carb is simply worn out due to age.
I don't really want to buy a new carb (unless I really have to) as I'm planning on selling the 250 and putting the money towards a nice example of a 350d. OEM is my last option really.