Overhead-valve four-stroke engines run on 110-octane propane, and are eco-friendly with zero evaporative emissions, because propane, unlike gasoline, is not listed as a marine pollutant by the DOT. Propane is 50-times cleaner than gas, with 96% less carcinogenic compounds, 60-70% less smog-producing hydrocarbons and no Class I or II ozone-depleting chemicals. It s also domestically produced, reducing our dependency on foreign oil. 9.9hp and 15hp models connect only to remote tanks and are complete with a 10' hose assembly