Well, I don't know your starting procedure, but if you are priming/choking before you're actually starting, you might be bringing too much fuel into the engine. If the tank is a bit high, it could just flow into the engine. My first trainer was a SIG Kadet Senior, its tank was a little high and I ended up using a clothespin on the fuel line to prevent flooding. Once started, I would just take the clothespin off. It ran fine from then on.