Good link for carburetor conversion kit for Brigs and Stratton engine.
http://www.propane-generators.com/
Converting to propane good on how to
http://theepicenter.com/tow102899.html
METHANE
Methane is a colorless, odorless gas with a wide distribution in nature. It is the principal component of natural gas, a mixture containing about 75% CH4, 15% ethane (C2H6), and 5% other hydrocarbons, such as propane (C3H8) and butane (C4H10). The “firedamp” of coal mines is chiefly methane. Anaerobic bacterial decomposition of plant and animal matter, such as occurs under water, produces marsh gas, which is also methane.