Single-cylinder experiments were conducted with a 3-valve carbureted pre-chamber stratified charge engine in comparison with a conventional engine. The pre-chamber engine operation is governed by many design and operating variables. This investigation was limited to determining the effect of overall air/fuel ratio, ignition timing and EGR on emissions and fuel economy at a single road load test condition.It was found that, as for the conventional engine, these operating variables are also significant for the pre-chamber engine and that a compromise must be made between good fuel economy and low emissions. The main virtue of the pre-chamber engine was found to be the ability to operate at leaner overall air-fuel ratio. This resulted in lower nitrogen oxide (NO) emissions than the conventional engine without EGR. The unburned hydrocarbons (HC) were found to be higher for the pre-chamber engine up to the conventional engine lean misfire A/F ratio.Exhaust gas introduced into the pre-chamber was found to reduce NO emissions significantly without a large corresponding increase in HC emissions as observed with the conventional engine.Only at very low NO emissions with severely with severely retarded spark timing and/or high EGR rate did the pre-chamber engine show a fuel economy advantage over the conventional engine.As the test program was limited to one load and speed, the results should not be construed to be typical of all modes of operation.