70 hp 1975 evinrude warm up lever kills motor after idle and dropping the lever.?

Went to drop the boat in today and for some reason when u let off on the warm up lever the motor stops? Didn't take it away from the ramp but what could be the deal? I know if ya drop the lever quick and put it in gear it will go but didn't want to take the chance and get stuck paddling lol. Anyways its an old ski boat with a 1975 70hp motor it starts and runs fine hooked up to the hose in the yard. This was the first time putting it in water any suggestions? Thanks in advance.