There might be a small screen in the exhaust (it's a spark arrestor) that gets plugged with carbon. Will cause loss of power and eventually the machine will not run. You can just take the screen out and it will run better. Check the exhaust ports (look into the cylinder area after you take off the muffler) and see if it/they are plugged. If so, use only plastic or wooden items to clear the carbon out. Be careful not to get any in the cylinder. If you do, pull the starter rope several times to blow out the carbon.

Otherwise, some fuel line cleaner would be a good try to assist in the problem.