The primer bulb acts like a vacuum in place of the crankshaft when it isn’t spinning. It pushes air out of the fuel lines and sucks gas into the carburetor. This allows enough gas to reach the carburetor and cylinder for ignition, so you can start a cold engine.
Clean Gas Tank
When the primer won’t hold fuel in the bulb something is either blocking the flow of fuel or air. Most likely, the gas is getting blocked at the source. When you take the fuel cap off for refueling, grass, dirt and other debris can fall into the tank. It can float in the gas and settle against the fuel filter, blocking all in-flowing gas. Drain all the fuel from the tank and clean it out with a brush and rag. Rinse the tank with a little un-mixed fuel.
Replace Fuel Hoses
The primer bulb attaches to the fuel hose system. This system connects the carburetor to the fuel tank. The first hose, the suction hose, draws fuel up from the tank. The second hose, the purge hose, expels excess fuel back into the tank. The primer bulb is attached to the second or purge hose. Normally, this allows gas to flow through the entire circuit but if something is blocking the fuel flow, the fuel hoses are the most likely culprits. Bad or old gas can clog the hoses to the point that all fuel stops flowing.
Replace Primer Bulb
If gas blockage is not the problem, check for air blockage or leakage from the fuel system. Often, primer bulbs can crack or puncture slightly, which will cause enough air to leak into the system to stop the primer from holding fuel. Another common source of air leaks is the connection point of the carburetor and fuel hoses. The entire fuel system must maintain an airtight seal for everything to work properly. After replacing the fuel hoses, replace the primer bulb as well.
Check the Carburetor
If the primer bulb still won’t hold fuel after cleaning the tank, replacing the hoses and bulb, then move into the carburetor. Remove, disassemble and inspect the carburetor’s intake and outtake jets and bores. Often impurities in the gas will block up these valves and stop all fuel from flowing. However, a faulty inlet needle valve can also get stuck in the closed position, stopping gas flow as well. Leave most major carburetor repairs to a professional.