make sur ethe field bias resistor under the reg is not shorted to case or touching chassis.
now disconnect field wire at genny and vreg.. bend the wires back.. use a jumper wire to make the connection.. this will rule out a bad wire in the harness.
if it still maxes out the charge.. field circuit is closed.. may even have welded contacts.
if vreg is old / non returnable.. pry the can off and see if the contacts are welded shut. many times you cn pop them apart and dress them.
if they are just staying closed but not welded, and no adjustment helps.. it will be time for a new vreg.
in an emergency, if you needed some charge, find a ignition resistor or light bulb ( turn signal may work ).. and wire it from ground to field on genny.. expirement to see what you get for charge rate. just unhook the wire at shutdown. if you have a 3 position switch, center off.. SPDT and 2 different resistors.. you could wire field to the center common post, and one other post thru large resistor to chassis, and other post thru small resistor to chassis. center is no charge.. with large resistor selected.. low charge.. with small resistor selected.. hi charge.
you just made a tickle switch.