If the engine will run and run well throughout a full tank one time, but not start another time, your difficulty is really not with the engine, iteself. You're either flooding the engine or letting it be dry of fuel. It's down to technique. You should set yourself the exact, same procedure each time you start the engine. Make sure it's not flooding between flights. Many engines are mounted such that fuel in the tank will siphon into the engine. The engine will be too rich, and become very hard to start. One way to prevent siphoning after the engine has been stopped is to open the throttle to full, put a finger over the carburetor air intake, and slowly turn the engine backwards. This will force the fuel in the fuel line back into the fuel tank. When the fuel line is full of air, fuel in the tank cannot siphon into the engine. You do need to make sure that there's a small loop of fuel tubing that is higher than the level of the fuel in the tank for this to work.