Based on my experience, a muffler that is choked with carbon can cause slow, sluggish performance. Remove the blade and the blade collar, then remove the muffler. Shake it a little bit to see if there are bigger carbon chunks floating around. As long as you have it off, check the exhaust ports on the cylinder and see if they are blocked with carbon. The next step I would take is to see if the air van governor is operating freely. It might need some adjustment to the spring tension. Sometimes I have removed the whole carb and have found that the tension spring has been installed improperly or the adjustment collar is set wrong. You will need a service manual to walk you through the proper steps. If you do not have a manual, there may be another member here who can direct you to one that can be downloaded.
Don’t give up. Your model has an F engine with a simple air vane governor, which I feel was Lawn Boys’ best design. When an older Lawn Boy does not run properly, it’s usually a small problem. The 2 cycle engine is very basic and with time and patience, most running problems can be easily solved.
