Is there any slack in the connection? If not, try expanding the turnbuckle connecting the throttle lever to the accelerator cable. Otherwise, turn out the fuel screw or reindex the throttle shaft.
Some people start grinding at the linkage to get the idle down with the fuel screw way up. I think I'm about there.
1989 D250 2wd. Scheid 14mm VE pump and 5x25's, J&H Performance 47RH trans with Suncoast manual valvebody, dual wastegates, 62/65/14 S300, NX Dual Stage nitrous, 487 on fuel, 972 on the jug.
another way to get really low idle is to use pliers on the cast ear that bumpstops on low idle screw then use linkage to set idle
learned this by accident the cast ear did not bend like i wanted it to duhh
93' w350 extended cab drw getrag, pump turned up and twisted, skirted flatbed 93 w350 extended cab drw auto, flatbed