That's what I'm trying to figure out. I have a new LX, all stock, and it seems to be on the ragged edge of 4-stroking. Sometimes it runs great, with a smooth WAAAAAAAAA sound, and sometimes it has a harsh tone and more vibration. I ran it at WOT for a mile with the harsh tone, then immediately checked the plug -- it was brown. Can anyone tell me what actually causes 4-stroking? Is it a mixture too lean, or too rich, or either?