Small Engines (Lawn Mowers, etc.): I have a single cylinder four..., adjustment screws, fuel tank

Question
I have a single cylinder four cycle string trimmer that I have been using for about five years.  Yesterday when I cranked it up I used it for about 20 minutes or so and abruptly the engine sounded different and the power decreased.  I would call it a blah sound and I noticed that it doesn't run smoothly now... jiggles like it might be mis- firing on occasion.
It seems to crank up with no problem but when I give it gas the carburetor makes that blah sound.  Would you have any idea what could be causing this.

This is a Ryobi 4 cycle 490R string trimmer.  The year would be 1999 or 2000 approximately.  No repairs have ever been made on this equipment.

Answer
From what you have described, it sounds like one of two things.  If equipped, start the mixture adjustment screws at 1 1/2 turns out from lightly seated and see if the problem is, at least, partially corrected. If no change, the carb will need to cleaned thoroughly (especially if you left fuel in them over the winter). Another possibility is a problem with the fuel tank vent (open the cap slightly to allow it to vent to see if this corrects).  If it is the tank vent, you will need to clean or replace it (usually integral in cap).  Hope this helps.