Whoever suggested that they might be overfilling the tank is likely correct. Most off-road tanks are sealed with an O-ring, and depending on the brand, the O-ring may be positioned partly into the filler neck of the tank. If you fill the tank to the absolute top including the filler neck, then snap the lid shut once you're finished refueling, the "plunger" effect of the O-ring forces enough fuel into the carb that it makes the engine run rich until you can clean it out. This is quite common and, as already mentioned, most fuel guns position the vent tubes about half way into the filler neck to stop filling the tank before it reaches the absolute top of the filler neck. I've never needed a pressure chamber on the exhaust pressure line because it's not an issue when you don't overfill the tank.