If an engine is mounted so that the needle valve is higher than the level of fuel in the tank, no model engine will "hold" suction in the tank. Fuel will always run back to the tank when the engine's not running. If the needle valve is mounted lower than the level of fuel in the tank, fuel will always tend to siphon into the carb and drip out if the engine's side- or inverted-mounted.

The ideal tank height is with the centerline of the tank level with the needle valve or no more than about 3/8" lower. This applies to just about every engine .90-size and smaller. Larger engines may need to have the tank closer to the needle valve centerline because they usually have lower fuel draw.

Whenever the fuel level is higher than the needle, and the fuel lines are full of fuel, then fuel will siphon into the carb. You can prevent this by having the fuel tubing make a small loop above the level of the top of the tank. This, then, will prevent siphoning if you make sure the fuel lines are empty after running. You can do that by putting your finger over the air intake and gently turning the propeller backwards. This will put a bit of positive pressure in the line and push the fuel back to the tank.