Well having to pump the prime bulb to keep it running it is a sure sign it is starving for fuel, and it could indeed be in need of a rebuild kit. If you can't get the low end needle rich enough to stay running then that does make a person think your pump may be very weak or not pumping at all. Air in the line would certainly mean you are not getting fuel to drawl properly as well. However its not quite the same as a  nitro engine that develops problems from an air leak in the tank or line somewhere, because your tank is vented and not pressurized from muffler pressure like a glow engine ,but instead is designed to pull its fuel from the tank which is where the prime method I mentioned before comes into play. Anyhow..I wish I could be of more help, but perhaps you are on the right trail and may want to take a peek inside the carb. Although I don't think it is absolutely nessasary, but since you mentioned getting a new spark plug anyhow....might I suggest that since you allready plan to buy a new one that you may opt to get a resistor plug if your auto store has any in stock. They are a bit more money however and I don't use a resistor plug since my radio gear is far away from the engine, but a resistor plug can save you from possible interferance created by the mag and when I decide to buy a new plug at some point I will probably get a NGK or Champion resistor plug.