McCulloch 2-cycle chainsaw won't fire, even with starting fluid sprayed in the cylinder. Verified that there is good spark at the new plug. Can feel the spark through the boot when plug is installed, so I know it's not bad spark wire. It was running and working fine prior to stopping suddenly. Haven't been able to get it to even fire since then. With good spark and with starting fluid in cylinder, what are possible causes of not firing?