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Misfire and cycle-to-cycle combustion variation are both serious problems in securing good engine performance and low exhaust emissions in the case of using extremely lean mixtures. Making some modifications in the ignition system and in the combustion chamber, and increasing the mixture turbulence, we examined their effects upon the lean limit, the engine performance, and the exhaust emissions. It was found that gap width and gap projection of a spark plug and spark energy as well as mixture turbulence had a great effect on extending the lean limit and improving engine performance with lean mixtures. A compact combustion chamber is preferable for lean mixture operation. Smooth operation of the engine can be maintained even at retarded spark timing by applying the above-mentioned items and providing hot intake air to the engine. Consequently, exhaust emissions, including hydrocarbons and oxides of nitrogen, can be substantially reduced.