Re: 7.5 mercury (no water pumping)
It doesn't matter what way the vanes on a new impeller were pointing when you installed them, as soon as you run the motor they'll curve around the right way anyway.
You might want to pull the lower unit again and try running a water supply up the water tube. You'll know for sure if water will circulate in the powerhead. You can also blow air up the water tube to see if it forcefully blows out the water discharge in middle of the exhaust tower (aka center section or 'leg'). See if you get any discharge out the telltale.
Couple of things that keep these from circulating water if the pump is good; squeezed-off upper water tube grommet, which requires removal of the powerhead to replace; and silt buildup in the block's water passages. The latter is fixed by removing the back cover over the spark plugs (this is not a cyl head, just a water jacket cover). Then clean out accumulated deposits.
Note the bolts holding the cover on may be stuck, use PB Blaster and careful application of heat from a propane or MAPP torch to help loosen these bolts. Don't force or they will snap. Caution PB is flammable so wipe away any excess before torching!!!
Something to do before 'biting the bullet' and removing the powerhead would definitely be taking off the back cover, then putting water up the tube to see if it comes out the block. Also a good way to flush 'trash' out of the cooling system. If you get no water out of the block this way, it's a sure bet the powerhead grommet is squeezed-shut.
To pull the powerhead, disconnect any interfering wires/hoses/linkages on the motor, and the powerhead retaining nuts around the lower circumference of the block. Around 4 per side if I recall.
HTH & let us know how it turns out..........ed
