You aren't using a 12 volt battery straight on the glow plug are you? They're designed for about 1.2 volts. You can use a resistor to drop the voltage, any OS engine manual will tell you how to do it. If you hook a glow plug up to 12 volts though it's only gonna last for about a second, but it'll be mighty hot!
Try soaking the carb with liquid wrench, tapping it gently with a plastic screwdriver to more the oil around, then working it. If that dosen't work, then try heating it with a plumber's propane torch. IMPORTANT NOTE remove all needles/O-rings and other plastic parts before heating! Just try and heat the outside very gingerly, the object is to expand the carb body without heating the barrel. Oh, and liquid wrench does burn, so try and get it wiped down first.