A few things to consider, restriction in the fuel line like a clogged filter? Did you clean the screen in the input to the carb inlet valve? Is the choke closing completely? Gaskets and diaphragm in the carb assembled in the right order? Inlet valve sticking closed? Inlet valve control lever set too low? A good test for a carb after before installing it, is to pressure test the inlet fuel port and if it hold 8psi like it should, bring a running shop vac nozzle up the the throat of the carb and the vacuum should pull the main diaphragm down and open the inlet valve, causing the pressure to drop. Just because diaphragms are intact doesn't mean they are working, alcohol in the fuel and age will stiffen them and make them nonfunctional, first step in carb issues is a COMPLETE cleaning and replacing both diaphragms and gaskets with a repair kit. Your most likely problem is just stiff diaphragms.