My 8403 has been running weird lately. When I start it, it runs good, but slowly loses RPM, then slowly regains its speed. I know the carb is good because I just cleaned it, including the jets and passages. I thought it might be a plugged muffler and clogged exhaust port, but I just replaced the muffler with an NOS cast one, and I cleaned the exhaust port while I had it off. I'm wondering if it needs a new ring and piston. It's a 1984 model, so it''s possible that 35 years of use has taken its toll. Did the F Series engine cylinders have nikasil? On that note, has anyone ever used a ring and piston kit from Ebay, or is there a better one out there?