They will overheat in a barrel after a while as bubbles of exhaust get mixed in the water and lower the efficiency of the water jacket. It should be good at an idle as long as the pump rotor is tight on the eccentric it will supply plenty of water. Those models had Oillite bearings on the prop shaft which are semipermeable. Some oil additives will cause the rotor to swell. This decreases the movement and volume of water. It there are ANY gaps between the eccentric and rotor, replace it. Space on the OUTSIDE of the rotor is good, that is where the water is. . . 🙂
