Well, you said it starts fine after it's been warmed up, so I'm guessing that the compression is OK. They do need a minimum amount of compression to start. The coil must be pretty decent too. Normally if they are weak, it shows more when they are hot.
Have you verified that the choke cable and choke are working correctly?
Have you tried different pulls of the choke when trying to start cold... like halfway, all the way out?
Have you pulled the plug right after trying to start it to see if getting too much, or too little gas?