This rule revises the Phase 1 carbon monoxide (CO) emission standard for Class I and II new nonroad spark-ignition (SI) engines at or below 19 kilowatts. Today's action increases the CO standard from 469 grams per kilowatt-hour (g/kW-hr) to 519 g/kW-hr. This action addresses the CO emission difference between oxygenated and nonoxygenated fuels that was not reflected when the Agency previously set the CO standard for these nonhandheld engines in a final rule published July 3, 1995. This correction of the nonhandheld engine CO standard will ensure that the CO standard for manufacturers of Class I and II small SI engines used to power equipment such as lawnmowers is achievable and otherwise appropriate under the Clean Air Act and that it is technically feasible for manufacturers to certify their engine models to the Phase 1 emission standards and make them commercially available for the 1997 model year. In addition, today's action permits the use of open crankcases in engines used exclusively to power snowthrowers. This change will allow engine manufacturers to certify engines with open crankcases to be used in snowthrowers upon a demonstration that the engine still meets all applicable emission standards.