I purchased a 1973 7.5 HP Mercury Thunderbolt outboard enginge about a year ago. I have not tried it in the lake yet, but today I set it up in a small drum of water to test run it. My question is, should the water pump run all the time (no matter what speed)? It seems to be putting out a good steam of water, but when I slow it down to a low idle or to shift in either forward or reverse it just about stops flowing until I speed it up. Is this normal or a sign that my water pump may be going bad. Any help would greatly be appreciated.
Agree with the above plus a couple of thoughts for you.
I would replace the water pump every couple of years, particularly if it sits up. Rubber vanes get stiff and this is cheap insurance.
And there is a hole for idle relief exhaust just below your lower cowling in the rear that tends to plug up with exhaust debris/oil over time. When that hole is plugged, then too much exhaust goes out by the prop, & bubbles up into your water intake which is underneath your anticavitation plate. If you can squirt some WD40 or rod out this hole then it greatly helps this problem which is particularly noticeable when running in a barrel. When this hole is open as designed, you can see some exhaust coming out. Also, as said above, you want to have a water stream always coming out.
Hope this helps and good luck, great little motors!
Symptoms point to an old impeller. Especially since the motor's sat for a year since you bought it.
Unless you know that the impeller was new or nearly new when you got it, it's likely that the blades are curled up on the "short" side of the pump and have taken a 'set', such that they won't straighten out when they get to the other side of the pump.
Therefore most of the pumping efficiency is lost.
I'd pull the lower unit and check it out, for peace of mind if not any other reason. Impellers are definitely cheap insurance compared to the cost of overhauling a fried powerhead or replacing a dead motor.