2000 mercury 75 horse wont idle or run trimmed down but runs and idles trimmed up

problem in the idle settings of the carby
could be rubber bits from deteriorating rubber hoses /water in the circuits but it needs attention from accredited mercury service centers

The idle air control valve is designed to adjust for the load on the engine to maintain the idle quality. When it's cold out, it's more important that the engine temperature sensor is working correctly so that it can trim the fuel richer to compensate for the cold running conditions. Also, a dirty Mass Air Flow sensor could give you problems with your idle. A shop could easily diagnose the poor idle quality for a reasonable fee if you don't feel like throwing parts at it again. Have you experienced unusually poor fuel economy or better than normal fuel economy this winter?

make sure you have fuel pressure first

Could be a vac leak, or a dirty throttle body, or a faulty idle air control valve. The computer controls idle speed by regulating the amount of air into the intake. Could also be something pulling on the throttle cable such as the cruise control.

It is likely the fuel pump is defective. It never produces the correct pressure based on the readings you presented here, it should be 42-44 PSI at idle and then when you throttle the engine it should go up to about 50 PSI, never down. To change the pump you must remove the gas tank from the car, this takes special tools (fuel line release tool) and a helper for sure. About 3 hours on the ground. This is a guess on my part and I caution you to have it checked out by the dealer before any parts are replaced.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control

More than likely, yes. If the regulator is controlled by a vacuum feed, try removing the hose from it and run the engine. If you feel that it will run off-idle after you've done this, replace the regulator and be on your way.

sounds like the reverse sensor switch is stuck in lock mode
check the switch

Clogged jets in the carb...clean out carb or rebuild carb.