high speed idle

[maxwell t]

up date on maxwell t post 5/19. My 2002 Jonhson 150 hp engine runs at very high speed and spews out black smoke when it should be idleing. Ran perfect on another boat one day earlier. Changed nothing but ignition switch. Replaced old switch and still runs wild. 3,000 to 4,000 rpms. Throttle cable is installed correct. All wires check out good. Can anyone help. Thanks.