If you read the instruction manual that comes with your chainsaw, you will probably see a note buried in the fine print, "Use Gasoline Only".
Many plastic parts swell up when exposed to the ethanol in common gasoline blends.
I have a Poulan and the gas cap becomes a stinker to get off and screw back on. Replacement caps are available from here. I have a spare set and swap them back and forth. One set is on the chainsaw while the other set is "on the wagon drying out."
Another solution is to find a station that sells "Pure" gasoline.
====> Link to find Pure Gasoline <=====