distributor hold-down prevents timing adjustment?

Also check the cam timing , the chain may be stretched . Thus retarding the timing and causing a rough idle.

Remove the distributor cap and watch the rotor as you move the crankshaft clockwise until the rotor starts to move then rotate the crank counter clock wise and note how far it moves before the rotor starts to move again. If you have more then 5* its time for a new chain.