New water pump but not pumping over 2500RPM

I have a new water pump instal

I have a new water pump installed on a 1990 140HP
Johnston it pumps pently of water at low revs.
However when you take it over 2500 RPM the water
dies off to a trickle and the higher the RPM it then stops pumping water all together. Any suggestions on why?

"Brett, could be a bit of crud

"Brett, could be a bit of crud/old impeller in the waterpump itself or the water-tube and the increased pressure from the higher rpm is causing it to clog.

Or, more common, a seal was missed, the key slipped etc and ditto, the increased water pressure is causing it to (leak or not pump) in the lower end - water will take the easiest way out, and the lower unit is easier than pumping all the way to the powerhead.

A pain in the butt, especially on a motor that size, but usually the first step would be to pull the lower unit again and inspect the work..."

"hi .. I had the same problme

"hi .. I had the same problme on a smaller motor..
Its one of two problmes, One the key has slipped r the second which i also had .. believe it or not there can be a slight difference between the inside diameter of an impellor. I got two at the same time from the Johnson dealer.. the first spun all over the place didnt fit snug even with the key. It pumped at idle like a dream but them at hig revs .. in the water it did nothing. so just take a look. and see if the impellor is snug,"