Check to make sure the primary throttle blades are completely closing and that the secondary throttle blades are just about closed. Check the throttle linkage, make sure the primary shaft isn't somehow stuck on the fast idle cam (choke mode). Any plugs/caps left off or open on the carburetor/intake manifold?
May God's grace bless you in the Lord Jesus Christ.
'78 BRONCO: 508" stroker, TFS heads, Dominator MPFI & DIS, cold air induction, dual 3" exhaust/Magnaflow mufflers, 100HP progressive dry direct-port NOS, A/C, Lentech Strip Terminator wide-ratio AOD, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, custom traction bars, 4" lift, 35" mud tires