Pull the carb off flip it upside down.
The weight of the float should hold back 14 psi ( atmospheric pressure , sort of ) , but usually 10 PSI is good enough
If it does not then there is some thing wrong with your carb
If it does then there is something wrong with your mower
remove the fuel cap
if that fixes the problem check the tank vent
Some time between 2000 and 2010 all states EPA regulations changed so the vent has to go inside the engine, usually to spiggot in the manifold
Some times these can syphon fuel into the engine