The Z-car engine is designed to operate on regular leaded fuel (1970-71), or on regular, low lead or unleaded fuel (1972-79), with the exception of 1975 and later vehicles sold in California, which have a catalytic converter. Converter equipped vehicles must use unleaded fuel. This also applies to all 1980 and later ZX models, since they all have a converter. The use of leaded fuel will plug the catalyst, rendering it inoperative, and will increase the exhaust backpressure to the point where engine output will be severely reduced.
Use of a fuel too low in octane (a measurement of anti-knock quality) will result in spark knock. Since many factors affect operating efficiency, such as altitude, terrain, air temperature and humidity, knocking may result even though the recommended fuel is being used. If persistent knocking occurs, it may be necessary to switch to a slightly higher grade of gasoline. Continuous or heavy knocking may result in engine damage.
Your engine's fuel requirement can change with time, mainly due to carbon buildup, which changes the compression ratio. If your engine pings, knocks or runs on, switch to a higher grade of fuel and check the ignition timing. If the engine requires unleaded fuel, sometimes changing brands will cure the problem. If it is necessary to retard the timing from specifications, don't change it more than a few degrees. Retarded timing will reduce power output and fuel mileage and will increase engine temperature.