Gpar,
I'll take a shot at your problem.... It sounds like a possibility that the engine is getting hot. Once it gets hot you may be "chasing the needles" and you won't get a good setting on the carb no matter how hard you try and your engine will run like garbage. If you start the engine and it runs good the first few minutes but then everything gets worse this could be a sign..... Set the needles when the engine has been run for about a minute or so and leave it. Also make sure you have good air flow to the engine... Bill