This paper presents an experimental research into the use of LPG in converting spark-ignition four stroke outboard (experimental) engine. Engine was modified to operate either on gasoline or on alternative fuel. Two different methods were adapted for operation with LPG; the first method is using vacuum produced by engine which supplies a constant mixture to the engine carburetion while the second one is accomplished using fuel injection (LPG). The results obtained indicate that with the use of injected LPG; torque, engine brake power and brake specific fuel consumption were lower compared to gasoline, while for vacuum system are higher except brake power. On the other hand, the carbon monoxide (CO), carbon dioxide (CO2) and nitrogen oxides (NOx) emissions were less in LPG mode as compared to gasoline mode while the higher hydrocarbons (HC) emissions were obtained. © 2011 Jordan Journal of Mechanical and Industrial Engineering. All rights reserved