I have a L120 with a 20hp Briggs it sounds like it is running on 1 cylinder until 3/4 throttle to full you can here the difference. I had it running just off idle took off the air cleaner cover, poured a little gas in the air inlet that wasn't firing and it fired right away, Is it possible to have a clogged jet or something? I replaced the fuel filter, and found the fuel lines a bit suspect as they are hard and brittle. It has 343 hours on the engine, and is a 2005. Thanks