Your bike has 2 separate but related problems.
The first is the petcock. In the OFF position no fuel should flow. Please show a picture of the petcock, as there are several different types, but the bottom line is that the petcock needs to be rebuilt or replaced.
The second problem is the carbs. The carbs have float valves which are designed to stop the flow of fuel when the fuel reaches the correct level in the fuel bowls. Clearly, this is not happening or the carbs would not overflow into the airbox. There can be a couple different causes of the overflow problem. One is that the float valves are sticking open or not closing tightly and thus allowing fuel to continue to flow even after it has reached the proper level in the carb bowls; another is that the fuel level in the carbs is set too high; and another is that your overflow tubes in the carb bowls (if your carbs have them) are cracked or otherwise leaking. Each of these problems is easily fixed.
Since you had fuel in the airbox it is likely that fuel also ran forward through the carb throats, into the cylinders, and down into the crankcase. So by all means check the oil for traces of gasoline and fix the fuel leaking problem before changing the oil or running the engine. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)
