15hp Yamaha dies in idle and cant get full speed

[Gatorzone19]

I have a 1991 15hp Tiller Yamaha 2 Stroke outboard. In the past couple of weeks it has been running really slow in idle and will die in idle if not revving it in idle. Also when i turn the handle to go full speed it will only go like 3-4mph. So i tilt the motor up all the way and have to rev it full speed and then put tilt the motor back down while it is in a high idle. I then can get it run full speed until I turn off the engine. Once I turn it off it goes back to slow idle and will not run full speed unless I rev it out of the water again.

It seems that when it has no force on the prop it will go full speed or when the boat gets momentum but when its in the water and the boat doesnt have momentum it doesnt reach high rpms.

Anybody have any ideas on what is wrong with my engine?