this could be a few things, carb issue,no impulse from the crank case to operate the carburetor,bad fuel line,bad crank seals. But I would go after the carburetor first. I would remove,clean and rebuild. I would adjust the carb to spec and see if that corrected your issue. Oh another possible issue could be low compression,bad rings scoring etc.Very suprised you can still get that fuel cap, a lot of parts for these older Stihl saws are few and far between. The rebuild kit you will need to get is for tillotson part #rk-21hs.