See Figures 1 through 9
Every car, regardless of other emission equipment, has been equipped with positive crankcase ventilation (PCV), the most basic component of an emission control system. This device simply captures the crankcase fumes, which on earlier cars had been dispelled through the road-draft tube, and feeds them into the carburetor. From there they are fed into the cylinders with the gasoline mixture and are ignited. The feeding of the gases into the carburetor is regulated by the PCV valve. With this information in mind, it is easy to understand why cleaning and/or replacement of the PCV valve, as well as regular carburetor servicing, are musts for the maintenance of peak performance.
Until 1968, models were still equipped with a crankcase breather cap. Post-1967 models have a closed crankcase which provides a further reduction in pollutant emissions.