ok, ive had this same porblem with my jawa for like a month now, and ive elimnated jus about everything...
the problem is this, when its cold, it starts up fine, regardless pretty much of the mixture screw setting. it drives great for about 20 minutes, maxing out at 25. after this, it starts cutting out right at 20 and looses alot of power going up hills. i rev it on the stand at this point, and the high end is awful, it sounds like its not getting gas. (cutting out and misfiring) so i put it away and take it out 20 minutes later and it runs great for ten minutes. it sounded like a gas issue to me, but whenever i pull the fuel line of the petcock, theres a steady stream of gas.
-the compressions fine (120 warm, cold)
-it runs the same with the gas cap unscrewed
my dad thought it might be overheating, although the fins are clean... the other thing is ive been running it with out the air filter lately, the foam thing discintegrated and was getting sucked into the engine.
thanks for any suggestions!