I had a similar problem with a 503 except it started ok cold but was hard to start hot. After chasing rabbits without results a called Dwight Edmondson , who was a very sharp Rotax mechanic in Goldsboro, NC. One of the things he suggested was to check the stater. He said to check (as best as I remember, 15 or more years ago) by reading resistance with an ohm-meter across the plugs from the stater to the ignition modules. If the stater is good it should read around 300 ohms. Mine was way off. I replaced it and never had another starting problem. Also, if you have the pump type primer, the ones I've seen are pretty poor quality and need replacement pretty often. Some of my flying buddies had them and they didn't last very long. One more thought, floats. I had to replace mine because they didn't, float that is. Anyway, just some experiences I had. Let us know when you figure it out. Good luck. James Thomas