1995 200HP Mercury Blackmax

[Lenn]

Can anyone tell me how to fix my 1995 200HP Mercury Black-Max which is not idling. First let me say that the engine was running fine until three months ago I was coming home from fishing and the RPM'S drop to 2000 and I HAVE NOT BEEN ABLE TO USE THE BOAT SINCE. I change the fuel pump diaphragm, the idle-speed control, service the carburetors and then change both switch boxes without results. I don't know if the technicians are competent down here in Barbados but I really need my engine to work. Please help me fix this engine. Thank you.
Sorry, just want to say that the compression on all cylinders are good and we have spark on all cylinders.

[boats.net]

Run a compression test, check to see if have fire on all cylinders.