Besides the reasons stated in favor of a three line tank, such as less possible leak points and not introducing air into the line going to the carb when re-fueling, there is another reason I like three line systems. With a two line fuel system, you are putting the fuel in before the tank clunk/filter, and can introduce crud into the fuel line and between the filter and the carb. Any crud that gets pumped will actually be trapped on the wrong side of the clunk/filter, and will go straight to the carb when the engine is run. With a three line system, the clunk/filer and line to the carb remain closed, and all fuel is filtered before it goes to the carb.

This assumes you are using a clunk/filter like the Walbro felt one, which you should be doing....

AV8TOR