Re: 7.5 hp merc outboard problems cooling?
Sounds like the impeller needs to be changed. Plenty of info on this site, use the search feature.
Basically there is one nut underneath at the water intake screen to undo, and one other nut at the front of the lower unit. With the shifter in neutral, the lower unit drops right down once you've undone these 2 nuts.
NOTE: Before starting to work on any part of the motor, ensure BOTH spark plug wires are disconnected from the spark plugs. Better yet, pull the plugs too.
There are 2 nuts holding the pump body together, the impeller slides out. Don't lose the little key. If the impeller won't come out, split it with a thin chisel or sharpened old screwdriver.
You'll probably see that the impeller blades are all curled up and won't stretch back out to hit the sides of the pump, this is why it won't pump at low speeds.
If everything else looks unworn, just replace the impeller. Lube the blades with all-purpose grease before installing.
Put a dab of grease on the flat of the driveshaft and use that to hold the impeller key in place. It helps to mark up the shaft the position of the key, with a marker. Slide the impeller down the shaft, align the groove in the impeller with the key. Turn both impeller and driveshaft and 'screw' the impeller into the pump, while engaging the key. You'll know if you missed the key since the shaft won't turn the impeller, it'll just spin freely.
You'll likely have to practice a bit to get the impeller engaged in the key, it's an art!
If your driveshaft has an O-ring groove at the end, install a new O-ring. Lubricate the splines with waterproof marine grease (wipe off any grease that gets on the top of the driveshaft).
Reinstall the lower unit, engage the copper water pump tube into the pump on the way up. Rotate the flywheel a bit to engage the driveshaft.
Button it up and enjoy! That's it in a nutshell, any fine points I missed you'll find by using that search feature.
HTH.......ed