They don't use diesel fuel, but the lead that is added to the gasoline they burn for greater efficiency produces harmful lead pollutants, just like cars.
四行程雖不加機油，卻必須在汽油中加「四乙基鉛」來防震、防爆，提高鋅烷值以加強燃燒效率，減少汽油燃燒不完全就排放的問題，也因此和汽車一樣，會有「鉛汙染」。