Be worth your while to go
here and follow the instructions. There are a few .pdf's that should be your bed-time reading for the next couple of nights.
I see from the fuel section it suggests that ZZR250 has a vac operated fuel tap? Might be a different market/type. Does yours have ON/RES/PRI on the fuel 'tap'? If so, there is a vac line going to the 'tap' to operate a diaphragm valve widget. If this diaphragm gets a little tear (they do perish after time) fuel can then leak straight into the intake. And while running, will get sucked straight in...
Something to check. Try sucking on the vac tube. If you get a mouthful of petrol - Bingo.
(Mouth cancer in 10 or so years is an added bonus!)
Heh!
Oh and...
Be advised that a major fuel leak into the intake might well have put a fair amount of unburned fuel into a cylinder - this will have wept past the rings will have messed up the oil properties. Good idea to read the section on 'oil change' too!
(-8