Water flows from the water pump to and through the powerhead, then out through the cylinder head hoses to the thermostat. Remove the hoses, then run the engine. If water does not exit the cylinder head hose flanges, the water pump is not supplying water to the powerhead.
NOTE that with the engine positioned upright in a large plastic trash can, the water level must absolutely be at least 4" above the joint where the lower unit attached to the long exhaust housing so as to be above the water pump. The water pump is not self priming and will draw air other wise.
If running on a flushette, the water must be turned on full force!
With all spark plugs removed at the same time, what is the compression psi reading of each cylinder? Is there any traces of water in any of the cylinders?
If "all" of the above checks out to be normal... suspect the water deflectors in the water passages behind the cylinder heads (as follows).
(Water Deflectors)
(J. Reeves)
Water deflectors are actually lengths of 3/8" outside diameter rubber hose, installed between the top cylinder and any cylinder beneath it, and also between the bottom cylinder and the block. The purpose of the deflectors is to have the water follow a definite path around the cylinder walls. Unfortunately the deflectors between the cylinders will at times swell sideways due to either a previous bad overheating problem, or simply due to age and salt corrosion. This causes a water flow restriction which usually allows the water to cool sufficently at low rpms but not at the higher rpms.
This hose material can be purchased reasonably at any automotive parts type store if you care to make your own, or you can purchase individual deflectors at any Evinrude/Johnson dealership at a somewhat higher cost.
Removing and installing them can be a hassle at times, but not always. I use a sharply pointed scribe with about 1/4" of the tip bent at a right angle whereas I can reach in, jab the tip sideways into the rubber, then yank it out. It's necessary to clean the seating surfaces where the ruber contacts the block with a small rat tail file to eliminate salt deposits etc. When installing the new rubber deflector, coat the deflector and the metal surfaces with WD40 which will act as lubrication to allow it to go in as easily as possible.
Make sure that you insert something into that deflector area before cutting and installing the deflectors if you make your own so that you will be certain that they are the right length and also that they will be seated properly. Usually a very small amount will be left extending about the block sealing area..... simply cut the excess off with a single edge razor blade.
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Now, if that doesn't keep you busy until at least 10:30 am tomorrow, get back to me.