Merc 90 won't go into high speed with the cover on the motor.

[Mercury 90 Owner]

With the cover on the motor, the boat can't even get on the plane. When I take the cover off, it gets on the plane and goes at high speed. I have demonstrated this repeatedly. Has anyone seen this problem or have any ideas?
It's a 1997 2-stroke.

 

[flashback]

A bird nest in the cowling air intake? I've seen it before.

 

[Chris1956]

An exhaust leak into the cowling? My '93 Merc V6 has an exhaust relief tube from the powerhead to the outside world. If it leaks, exhaust gets into the cowling and then into the carbs. Exhaust has less O2. Less O2 means no power.