What is the usual time after start up for water to be discharged from this motor? Would having it at fast idle speed up the cooling or is there a thermostat controlling the cooling?
Do a search. You'll find several discussions on the cooling system for these engines.
It is hard to answer your question. I know on my 1982 150, I get a dribble (barely) after about 15 seconds. On my engine, water only gets to the tell-tale when the t-stats open. It will get a weak stream after running hard, but then it will reduce to a dribble again after idling a bit.
In general, I would say 15 seconds is a good number.
My '95 150's took about 30 to 60 seconds to have water come out of the telltale port. This really doesn't tell much, other than the water pump is working (sort of), so I wouldn't get my shorts twisted in any case. My Merc discharged from the end of the cooling cycle and the water was normally pretty hot coming out.
