I need help with my grass trimmer?

I took it to my local "authorized" dealer and he sat on it for about a month and then told me that I needed a carburetor, which was what he told me when I first brought it in. So I took it home. The manual leaves much to be desired. I changed the oil, cleaned the air filter and adjusted the valves. Drained the gas, cleaned the gas filter and refilled it with fresh gas. I checked the gap on and put in a new spark plug. It starts pretty well and It idles fine but when you try to accelerate it wants to stall. The only visible adjustment screw that is in sight is an idle screw. No mixture adjustment. Is there an internal adjustment or do I just have to change the carb? Most all the repair shops around want to sell you new equipment. Lots of profit and no work. The trimmer is about five years old and my yard isn't very big. I would have expected at least ten years out of it. I'd be grateful to hear from someone that really knows how to repair this stuff. Thanks ahead of time.