I like to run my small engines dry at the end of the season (like Pete C.). I worked for a small company that made material for carburetor gaskets (among other things) and we use to test gaskets in solvent to see how much they curled. I found that some gaskets are designed with U-shaped cuts in them so the flap acts as a check valve. If the rubber deforms in that area, the check valve no longer works properly and you have problems. My old boss liked to say that usually if you have a problem with an engine, it's probably related to something made of rubber.