Change if signs of wear are evident. The spark plug firing tip gives clues on engine condition. Look for worn or burned electrodes, sooty or oily deposits when inspecting plugs. Plugs should be replaced as part of a tune-up. Timing adjustment is an essential part of a tune-up.
Fuel filters trap dirt before gas reaches the carburetor.
A clogged filter starves the engine of gas. The fuel filter should be replaced as part of a tune-up. The air filter is usually replaced as part of the tune-up. If you can't see light through a dirty air filter, change it. A tune-up should include idle speed adjustment.