I have a Stihl 021 saw that has started and run with no problems from day one. I have been using it the last few days to clear fallen branches, the result of early snow storm in CT. This week I cannot get it to start. I have changed and gapped the plug, removed the spark screen, changed the fuel filter and fuel line, cleaned the air filter and filled it with fresh 50:1 fuel. It will start immediately with starter fluid, but stop as soon as it is burned off. It seems pretty obvious that the carb is not getting fuel, but I don't know what to try next. Suggestions?