You can buy a spark plug sand blaster for about $25 to clean the plugs. It certainly is cheaper then buying new plugs all the time. Turn the mixture screw in slowly till the engine starts slowing down. Then turn it back just enough to get back to the idle speed it was at. Check the float level in the carb. If it is too low it will run rich. I can't imagine why someone will idle an engine for 30 minutes except in a bad traffic jam.
