Hello John! Yes clarify "cranks" for us. I think you are saying it runs great when its cold then dies as it warms. By "cranks" I am guessing you mean firing. How are you operating the choke? If you have to put your hand over the carb, I am thinking the choke isn't sufficient to start it? Is the choke fully on when it starts to run poorly?

The start up process that seems to work well on a stone cold engine would be, pull the choke all the way out. Pump the throttle 3 or 4 times. Turn the ignition on and crank the engine. The instant it fires press the choke in to 1/2 way. Keep the engine running with the throttle.

Give us a little more information John, I am sure we can figure it out.