I Need Help!

[Jlang1221]

I recently bought a 1985 Ranger 320v. The boat has a recently rebuilt '79 115hp Johnson. This is my first boat, and I'm pretty pumped. But, I'm having problems with the motor. When tested on a boot, it runs great. Today however I took it to the river, when I backed it into the water, she cranked right up, but when we put it in gear. it dies. It idles fine,so i dont think it needs to be any higher. If anyone has had this problem please help.