1ABSTRACTSmall off-road 4-stroke SI-engines have extraordinarily high pollutant emissions. These must be curtailed to comply with the new Swiss clean air act LRV 98. The Swiss environmental protection agency (BUWAL) investigated the state of the technology. The aim was a cleaner agricultural walk behind mower with a 10kW 4-stroke SI-engine. Two engine designs were compared: side-valve and OHV.A commercially available 3-way catalytic converter system substantially curtailed emissions: In the ISO 8178 G test-cycle-average, HC was minimized to 8% and CO to 5% of raw emissions. At part load points, the residual emission was < 1%. Simultaneously, fuel consumption improved 10%. Using a special gasoline (Swiss standard SN 181 163), the aromatic hydrocarbons were curtailed, e.g. Benzene < 1%, and fuel consumption further improved. Those results were confirmed in field tests. The engine is approved for retrofitting. Disquieting are the high emissions of ultrafine particulates: the catalytic converter only minimizes the volatile but not the solid particulates.