Sounds like classic air leak symptoms to me and a bad one, I would check the crankshaft seals by pressurising the crankcase and using the soapy water, if it isn't this then the jug base gasket and carb gaskets. There is nice video of a pressure check on this site - think it may be one of Brads and is worth a look - would think your seal/seals are pretty far gone IMHO.
My jonsered 630 had a real bad crank seal, as soon as the saw started, it would run at half WOT, the only way I could get the idle down was to run the idle at super rich 2.5 - 3.5 turns out but it would run smokey and foul up. The idle adjustment could be left out and it would still fast idle.
It is worth looking at the carb and making sure the butterfly valve closes up fully but my smart money is on an airleak.
Good luck - let us know what you find
Spud