what will go wrong if the fuel filter is full? The amount of fuel insside the fuel filter can be full or just half of its capacity. So long as you have correctly installed the fuel filter with the nipple flow arrow pointing toward the direction of the carburetor, then that will be fine. the location of the fuel pump will play a role on the amount of fuel in the filter. If you have an electric fuel pump located before the filter, the filter will be full. If the fuel pump is suction type after the filter, the filter may or may not be full of fuel. If the filter is full and nothing comes out of it, then that filter is clogged.