I am trying to get a chipper/shredder running to no avail. The unit is a MTD model #645, the engine is a B&S 130212-3250-01. I took the carb and gas tank off, cleaned the carb and replaced the tank,and gaskets, After installing everything back in place, I tried to start the unit. It wouldn't start, so I pored some gas in the carb and it ran 'til it burned the gas in the carb, then dies. What pumps the fuel from the tank into the carb?:confused2: