1985 Kawasaki GPZ600 carby flooding engine when

Any gas engine requires fuel, air, compression and a spark at the right time to work. You have fuel, you have a spark, what you need to check is air (check your air filter - usually forgotten about) and if that is OK, check that you have compression. If those two are ok, check the timing of your spark (depending on the motorcycle involved, that could be adjusted using the contact points gap).

Check the shear key on the flywheel, the engine could be a little out of time but the key isn't sheared all the way in to.

if it has carbie/s then check for a faulty float needle and seat
may be a simple thing such as a hair stuck under the needle as it is flooding

I would say the problem is in your carbys, with the float levels, or a sticky needle and seat. If you open the fuel valve, fuel will flow into the carbys, ans shut off once the correct level is reached. I think the problem is in your carbys, not the fuel valve. I would remove the carbys, take the bowel off the bottom, and check the needle and seat, and float levels. It is not recomended to change the float levels from standard.

Sounds like needle & seat in carbys could be sticking or float levels maybe to high when you turn your key on your fuel pump starts and this puts pressure on needle & seat and if floats are to high the petrol just keeps flowing Float is what stops petrol flooding the carbies, check minor things first like dirt in needle seat if you have a compressor give carbies a light blow out.

If spark plug is wet with petrol then float level is incorrect either by dirt under float needle or set too high.

If you havent altered / removed the carb and it has just happened, somtimes gently tapping the float bowl can dislodge the dirt. Otherwise you will need to remove carby, (I assume it has one) and clean out.

the floats may be damaged
try to level the floats with the base of the top part of the carby and see how you go .
you may have to try a few times until you get it right .
Check your needle point too as it may be stuck as well .

check for carby piston stuck open or stuck needle & seat! & float level

Sounds like the float valve is stuck in the carby and allowing fuel to keep flowing.

Give the carby bowl a light tap with some wood to free up the float valve. You may have to pull the carby off and have a look if that doesn't help. Fuel cannot bypass the float valve in normal operation.

I don't know if its any help but I've fixed my Honda with similar problem. It turns out I reassembled the rich carbys air cut off valve incorrectly. Have you checked yours? I'm wondering if the cut off valves somehow affect the other parts of the carbys. Mine is running sweet now.