Why won't my Briggs and Stratton 4.5 HP lawn mower engine prime I have removed and cleaned the carb. The primer worked properly before disassembly. Now the bulb won't shoot fuel into the carb?

The primer bulb probably needs to be replaced unless either one of the fuel hoses leading in and out of the primer bulb bracket is clogged. Be sure you mark how each fuel line goes before you remove the bulb so you can replace them correctly. Hmmm. Thinking more about it: If the primer worked properly before disassembly, chances are you crossed up the fuel lines when re-installing. Try switching them.