The lean misfire limit air-fuel ratio of a spark ignition engine was extended by various modifications of the intake and ignition systems. The effects of long duration spark, extended spark plug gap projections and gap widths, and a vaned collar intake valve are reported. These modifications allowed for reliable operation up to air-fuel ratios of 24:1. The experimental apparatus and procedure used in this study are described. Conclusions are drawn concerning the optimization of the various modifications to extend the lean misfire limit and reduce the exhaust emissions. In general, all modifications extended the lean misfire limit, but increased gap width had the most profound effect. In all cases, the exhaust emissions were reduced by extension of the lean misfire limit.