Runs but doesnt rev up

Hello again, i'm not sure if this is a problem from my other posts but i got my briggs and stratton mower running after a year or being stored in the shed.. I prime the carb 3 times as instructed and the mower starts as it should but its not reving up as it should. It idles very low but it increases when i press the primer again for a short period then goes back to idling very low. I have put a new diaphragm on and nothing seems to be blocked is this a lack of fuel issue ?

Any help will be great thanks.

Runs but doesnt rev up