When you say the fuel tank is in the proper position does this mean in relation to the centre of the carb? In other words, is the centre line of the tank close to the centre of the carb? Has the engine been run and the main needle set properly? With these two things set correctly then syphoning is normally very slow. Syphoning can't be avoided with a full tank if there's fuel in the line to the carb so it's best not to fill a tank until immediately before you're ready to go fly.

You've got an excellent engine there so I presume it's had a lot of rich running in?