I am engine illiterate. I bought a mower that has been sitting for a year. The fuel was not drained and their is no shutoff valve for the fuel. The engine will not start. Guy sprayed starter fuel into the engine and it fired up to show me that it worked (I assume this is not a good thing to do). But, now that I got it home, it will not start. Does this mean I need a new carb? And are they simple to install, if so? Or could it be something else?