Priming is squirting fuel directly into the carburetor’s air/fuel intake stream, usually before cranking the motor. It helps with cold starts by providing an immediately combustible mixture.
Squeeze bulbs are commonly used. They have two one-way check valves so that squeezing the bulb forces trapped fuel out the outlet end. When released, expansion sucks new fuel in the through inlet check valve. Both check valves must work and all fittings must be tight.
Possible problems include loose or deformed fittings, improper installation, bad check valves or holes, . The bulb must be installed so the outlet end is towards the motor.
Membrane (aka pumper) carbs like the Walbros (image at right), can also be primed by air vent pressurizing. Blow in the fuel tank’s air vent line which forces fuel up to the carburetor under pressure. Insert a thin wire or other object into the needle release hole. That pushes a fulcrum which lifts the main needle valve allowing fuel to pass. Keep up pressure on the vent line while doing pressing and fuel will squirt into the carburetor throat. A WG8 carb is shown. Some have a button that sticks out so you don’t have to insert anything.
The last method is direct priming. Fuel is squirted directly into the fuel/air stream between the carb and engine. This is usually done by a pull/push pump where you pull the knob to suck fuel in then push the knob in to force the fuel into the carb. Be careful because if air leaks are present they won’t be obvious and will dramatically affect engine operation. If the fuel source is pressurized (even by gravity such as those with a fuel tank above) then this system could also potentially allow fuel to feed.