The simple answer is Ethanol. Since 2001 automobiles have been built to run on 15% Ethanol fuels. Small engines such as mowers and chop saws are not. The Ethanol actually draws water into the fuel system and wrecks your carburetors. The best reccomendation is to spend a few dollars more a gallon and get fuel at either a Marina (boats need non-ethanol fuel), or a retail store. Yes, it may be close to $8-10 p/ gallon, but it is still far less expensive than replacing a saw or a carburetor.
For more information, see Consumer Reports here (http://www.consumerreports.org/cro/news/2013/03/gas-with-ethanol-can-make-small-engines-fail/index.htm) or any number of other internet websites.