Change the plug and the air filter. Mine was bogging to and you would be surprised how dirty that little filter can get. If you've already done this then I'm at a loss, I don't think there is much adjustment on the carb.
If a plug won't work and the carb has no adjustment, like some are now, it could be the spark arrester is plugged or the exaust port is plugged up. Either of the last two are easy fixes. Clean the spark arrester or pull the muffler off and clean the carbon out of the exaust port. It will run like new again if that's the problem.
I would of said the spark arrestor, too, but Eric beat me to it.
If you still have your manual, it will tell you exactly how to set the carb. I have had very similar trouble with my Stihl hedge trimmer, and learned from a quick look in the manual how to reset the carb proper. It's worth a check if the spark plug and air filter are both still good, and the spark arrestor has already been replaced, cleaned or removed.