I don't know the LT500 at all but most older Suzukis had a vacuum operated fuel tap, on PRIme it operated like a normal tap but on RUN and RES the vac pipe from the inlet manifold opened the tap as soon as the engine is spun over. Unfortunately Suzuki decided that this meant they didn't need the overflow pipe from the float chamber, which other makers fitted in case the float needle sticks. So, if you left it on prime and the needle stuck, fuel had nowhere to go except airbox and engine. I would assume your new carb must have the overflow, so if the needle sticks, fuel can run from that.