Air filter. If the air filter is clogged or dirty, the engine may not start or may run rough.
Fuel filter. If the fuel filter is clogged, it will restrict the flow of fuel to the carburetor. As a result, the engine may not start.
Fuel tank vent (also called a check valve). The fuel tank vent allows air to re-enter the fuel tank. If the vent is clogged, the engine may stall.