The purge bulb works by drawing fuel through the fuel filter, fuel line and carburettor. Then it pushes the fuel out the return line into the tank. If the purge bulb will not compress, check the lines as they are likely to be connected backwards. If the bulb will not come back out, check the fuel filter, fuel line and carburettor for blockages. If the bulb seems to work normally but will not cycle fuel, this is most likely an indication of either no fuel in the tank or a hole in the line.