Has hard time idling

[16withaboat]

I bought a 1978 ranger fish and ski with a 1978 175 johnson outboard and im only 17 so i dont have a world of experience but i know more than most my age. But my problem is that my outboard runs fine on the muffs but dies when put in the water. i know its the back pressure killing it but i dont know where to start because i cleaned the carbs with no signifigant difference although i didnt get to the pilot/idle screw which i think was a mistake. But the exact way the motor is running is that it will start (hardly) when i have the idle lever lifted all the way up, and then if i pull it down and get it into gear and give it gas before the idle gets to far down and dies it will plane up. once i get it going on plane it will run till its empty or until i slow down then its destine to die unless i idle it at a good 1500rpm which cant be too safe. its start immediatly if tilt the prop out of the water where the exaust is free. so im thinking (cleaning idle screw, it has low compression, IAC valve, spark plugs) an old man said it sounded like it was missing when he was listening to it but we checked and all cylinders sparked.