these carbs use pressure pulses from the engine to pump fuel up from the bottom of the tank into that little bowl that the main jet sits in
The pressure pulses move the diaphragm up & down
Yours is leaking and allowing the pressure pulses to go directly into the tank
To check run the tank down to about 2/3 full then remove the cap
You should see bubbles rising to the surface if the pump is leaking.
The diaphragm must go directly against the tank then the gasket sits on top
Very common mistake is to fit them wrong way round.
I always hold the carb upside down , slip a couple of screw through it then place the gasket & diaphragm on the carb then the tank.
Do up the screws lightly to hold it all in place, invert then fit the remaining screws
Do them up in a diagonal criss cross mannar like you would do with a head till they are tight
Refit the carb & tank as a unit to the mower.