Check the oil for gas

You might wonder why I say look in the oil tank for gas when it's in the air box.  If the diaphram in the petcock has developed a leak.  It will pour into the intake thru the vac line.  Then it can either go into the air box or the engine.  Sooner or later in to the crankcase.

So check the vac line, should be dry.

Then check the float valve.  they sometimes get stuck.  give the side of the carb a rap with the plastic handle of a screwdriver.

put the petcock on prime
disconnect the vac line and plug both ends
if you still have the problem then not petcock.
turn petcock to on when you want it off.