Rather than the carb I would suspect the intake valve being stuck open. This happened once on a Craftsman 10hp we used to have.
Couple of things you can do,
#1 Drain the tank, pull the plug, crank until all the gas is out. Sometimes this will let the valve back down as it was just the pressure of the gas from the carb holding it open.#2 As in #1 but then put a whole bottle of carb cleaner into the tank with an equal mix of gas, put the plug back in and crank. This is on the assumption that the float valve in the carb is sticking also. Once the mower is running better add more gas to thin out the carb cleaner. #3 Pull the plug and air filter. Spray a bunch of spray carb cleaner into the carb and crank. Put the plug back, spray another bunch of spray carb cleaner into the carb and crank again. This method is unlikely to unstick a float, but might unstick a valve. #4 Pull the plug, add a hefty dose of Marvel Mystery oil to the gas and another ounce or so to the oil. Crank until cylinder is clear. Replace plug and crank again. Once engine is running good change the oil. I would resist fiddling with the carb if at all possible. To those who aren't very mechanically inclined these small engine carbs are very easy to screw up. I've messed up more than a few myself.
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