I'm trying to flush the cooling system on my 1989 mercury 8.0. I have it in a bucket of water. How long should it run at idle speed before I see water coming from the tell tail? I have only let it run for a few seconds for fear over over heating it, but in that short time have seen no water coming out. It is submerged over the top of the water inlet holes above the prop...before any one asks 
. Am i just not letting it run long enough??
. Am i just not letting it run long enough??