stillooking

i can't offer specific advice but perhaps you should check on the yamaha aircooled website if you can't get an answer here. as you said there probably will be issues about fitment as the 250 is a bigger frame bike etc

deffo

i don't think if they've got an o-ring on the needle valve, mine doesn't on a '72 cs5-e (mikuni) but he'll have to confirm

I'm not an expert, but I've been having similar problems with a CS5-e 200cc - judging by the forums it's a quite common problem. Have you removed and dis-assembled the carbs yet? If not, clean them internally and particularly inspect needle valve and seat beneath the floats. It might be sticking or worn, allowing the fuel to continually flow through and thus overflow. Also, check float height and verify against proper spec. I'm about to get mine going again to see whether this has fixed it but I just need to try and decoke some of the sludge in the silencers first.