My $.02 -
If the pump doesn't help, consider this:

My K&B Sportsters never liked any kind of pressure - they prefer to run from a vented tank. The trick with a vertically mounted engine like your Super Sportster is to keep the carb throat as close to the tank center line as possible. If you have room, try moving the tank as far towards the top of the fuselage as possible. Or you could turn the engine on its side, but then the cheeks look terrible.