1997 Mercury 250 EFI Offshore running rich and smokey!

Powerhead recently rebuilt. 25 hours on it. Thick exhaust coming out of propeller. Ocean water gets fuel and oil sheen on it when the boat is idling in the slip. Runs rough in low idle (900- 1000 rpms). Runs amazingly well at higher rpms (2000 plus). New plugs and recent service done on VST. Was running at high idle (1200 rpms) and I lowered to 900. Lower plugs have been wet with fuel/oil even after service of VST. Went hoop netting the other night. Ran great getting out to the spot but when we were dropping and picking up the traps there was so much smoke coming out of the engine that we all got major headaches. Ran great back to harbor at high rpms. Help! Thanks! Bill

Do a pressure test on the fuel system.
There is a schrader valve in the system.
You'll need a manual to compare readings.
My 225 was doing that.
The readings was supposed to be 36-38 it was 41.
The regulator was bad.
The whole world said they don't ever go bad. Mine did.