I just bought a YZ250 (91) and it slowly blows water out of the overfill tube. The person that owned it before me said he re-built the top and bottom end. Could a gasket be blown or is the radiator cap worn? The bike runs good.
Sounds like the cap could be bad. I have the same engine and it only throws the fluid out when it's really hot or when it's overfilled, and when it does it ain't a slow leak, it's a full force deal. And if a seal was shot, it'd just blow the coolant out the muffler and push steam out the overfill.
Another possibility is a head gasket leak from the cylinder side into the coolant system, which causes the system to pressurize and over flow. Watch the level of your coolant closely. If it continues, I would pull the head off, check the gasket for any signs of a tear or leak, and also check the head surface for warpage. Keep us posted.