1987 Merc 3.0 Alpha 1 - 140 HP.

Reinstalled the manifold with new gaskets, tightened everything up.

We started the motor and ran it dry for maybe a minute or two.

Finally figured out how to make a water barrel for the outdrive that would
actually work, filled it with water and re-started the engine.

We don't seem to be drawing any water out of the barrel. Engine temp rises
above normal, and we pulled the brass plug out of the bottom of the
manifold--no water coming out.

My questions:

1/ What's the possibility the cooling system is "air-locked"? ...and would
it be normal to have to prime it with water?

2/ How long could you normally run this setup "dry" (outdrive out of water)
before you damaged something? ...and what would be the first thing to go
(impeller?).

Thanks
Dean