if you are mounting the tank below the carb, the little fuel pump will probably not have enough suction to suck the gas into the carb. all the "fuel pump" is on those is a small rubber diaphragm inside a block on the carb. this is why they all have primer buttons because the tanks are not mounted on them in a way to where gravity can help feed the carb with fuel. notice that the fuel tank is mounted kind of at the same level as the carb. and not very far away from the carb. if you pump the primer bulb a few times, it floods the carb enough to get the motor started, and then once it's running, the engine creates a small amount of vacuum in the carb. which makes the rubber diaphragm move up & down. there are holes in the "fuel pump" block where the diaphragm is located and when the rubber opens and closes over the holes, it creates a bit of suction to help suck the fuel into the carb.
your best bet is to mount the tank higher than the carb so gravity will feed fuel to the "fuel pump". then, the "fuel pump" will only have to worry about getting the fuel into the carb. in this situation, rather than it trying to suck fuel up hill from a tank that is mounted below the carb.
