how is the o-ring on the fuel tank? how is the pipe internally. i know i had two traxxas plastic pipes melt internally. what could be happening is the inside is partially covering the outlet to the tank and only when you plug it does it have enought to force presure to the tank. also check the little nipple on the bottom of the tank. a piece of junk may be partially blocking the hole restricting flow or maybe something in the carb.