The fuel system has a filter to clean the air that enters the engine. It is important that air does not have a chance to bypass this filter through a leak. On engines with carburetors, there is a paper gasket that fits between it and the bottom of the air filter housing. A missing or damaged gasket can also allow unfiltered air to enter the engine.
Leaking vacuum lines or vacuum accessories can allow dirt to be drawn into the engine as it operates. Dirt is the number one cause of engine failure. A light is shined through a paper air filter to check it. Look to see that there are no small holes in the paper that would allow dirt to pass. Compare the amount of light that a new filter will allow to pass through the paper to give yourself an idea of whether an old filter is dirty or not.
Sometimes when an air filter is changed, dirt from the outside of the dirty filter is carelessly allowed to enter the engine. The equivalent of two aspirin tablets of dirt causes more wear than that caused by 75,000 miles of normal driving.