Stuck floats are usually the cause of petrol flooding outta the carbs. Float bowls off and see if the floats pivot on their pins ok ... they should move up and down freely. Are the floats the old soldered brass type on this bike?, if so make sure they're not punctured (and full of fuel)
Also check that the float needle valves aren't sticking. There are tiny springs inside them, check they don't stick and also examine the conical tips of each of the valves for any ridges or other signs of wear. Check the needle valve seats for gunk... anything that might make the valves stick.
Before takin' yer carbs off... you could try gently tapping all 4 float bowls with a bit of wood (e.g end of a hammer shaft), this can often free a sticking float valve mechanism