72 dart 318 problem please help

ok, 1972 dart 318 2-barrel stock . when driving down the road, and let off the gas , the engine does NOT slow down to idle , then after about 8 seconds it starts to lower its rpms slowly back to idle . it is NOT the linkage , the linkage is NOT binding up anywhere . when in park and i manually pull the accelerator . it behaves the same way . kinda like a run-on . this just started doing this yesterday. all i can think of so far is carbon build-up in the engine , i tried carb cleaner with NO results . anybody have any ideas ? please help -thanks