Mercury 7.5hp - Not pumping at low rpm

[sfmiller]

I have a 1978 7.5hp Mercury. I replaced the impeller and took it on the lake. Now it will only pump water when I am running it at least 1/2 throttle. Any lower and it stops pumping water. It used to do just fine for trolling but now I'll over heat.

Any suggestions on what might be happening?

Thanks,
Scott