Originally Posted by Army

I expect a '80 SD has a piston vacuum pump? If so you'll notice that the timing device has a rollercoaster-like track upon which the "wheels of the vacuum pump" run. If these tracks are not in a favourable position - so that means the vacuum pump's wheels are not in the dip - rotate the crankshaft (and thus the timing device) so that you don't have to fight against the return spring in the vacuum pump.