Fuel tank placement is important to insure proper engine run. The negative pressure created in the crankcase is used to draw fuel into the carburetor. The closer the fuel tank is the engine the better. If you can get the tank close to the same height as the engine that will help insure good fuel flow. The larger the engine generally the further back you can place your fuel tank because of the greater amount of negative pressure created in the crankcase. The pressure fitting on your exhaust is a must. Most engines will run very poorly without this added pressure to the fuel tank.