I am not familiar with the term "a lot of gas build up inside". Looking into carb throat and operating the throttle linkage, you should see the momentary gas stream from the carb accelerator pump. If so, gas is getting to the carb and the fuel pump is probably okay. With your symptoms of running for a couple minutes then dying (if no gas in the carb), I would first suspect a clogged filter either the one by the carb or the sock in the tank. But before doing a lot of work, the culprit(s) needs to be determined. Disconnect the gas hose from the carb and put the free end into a catch can and crank the engine some to see if gas enters the can. If no gas and there is a in line filter, pull hose from filter inlet, put it in the catch can and repeat cranking test. If still no gas, pull the hose from the fuel pump inlet, put it in the catch can, and pressurize the gas tank (I blow into the gas tank filler to avoid over pressurizing it). If you got gas at this step, it is likely the fuel pump. If not, keeping moving back to towards the tank to find the obstruction. Do not forget the vent system, crimped lines, crud in the carb, etc.. When working with the fuel system, keep a fire extinguisher handy to avoid further stress.