Have you replaced the air cleaner. A clogged filter will cause too much fuel to be used. Also does it seem to run OK?
That plastic unit is the carb. There is a rubber diaphragm under it that has gone bad. The part number is XXXXX you will have to remove the fuel tank and carb assembly. Split the carb from the tank and replace that diaphragm. Pay close attention to how the linkage hooks top the carb, so you can get it back on correctly.
You can get the part at any mower shop around you. Or follow this link and type in the part number. http://www.partstree.com/parts/