2000 Evinrude 40hp issues...

[Hookem_9]

Need some help diagnosing a outboard issue.... I bought a boat with a 2000 Evinrude 40hp on it early this spring. The only reason I bought it, was to put the motor on my 16' Alumacraft.

Some things to note:
The previous owner had the oil injection system removed, so gas is premixed.
When I removed the motor from the boat, I just cut the 2 wires going to the oiltank.
So, yesterday, I drop it in the river for it's first run. Starts great. let it warm up, still purring away. I take off. It's running great, WOT for about 20 seconds then starts slowing down, until it slows down to near a crawl. a little more than idle, but enough to plow my way back up river back to the launch. If I let it idle for a few seconds, then hit the throttle, it's full power again for a few seconds, then cuts down again. Thoughts???