When I start mine using the choke and put the choke in once it has started it takes about 8-10 minutes (depending on ambient temps) to warm up before it will idle correctly and will stall at idle before it has warmed. Have you tried a new coil, they can be fickle creatures? My comment above still holds, they are simple things to check and adjust and can make a difference. I also assume that the plugs colour and condition are correct?
