Mercury 150 hp 2 stroke 2003 everything runs fine with idle throttle lever engaged but when you put motor into gear i stalls idle to slow how do you adjust it thanks

Seems like your are describing a outboard motor. If so, the Boat section of Fixya may have more experts that you need. You would want to avoid turning any screws on the carburetor body. The externals on the linkage have some adjustments like throttle stops.

The linkage on boat shift mechanisms are tempermental. They usually combine 2 functions on 1 control lever. They are synchronized that when the transmission shifts from neutral to forward and neutral to reverse the idle speed is low. This keeps the gears from grinding and takes the shock out of the shifting process.

Another thing different than on a car engine is the effect of the propeller on engine rpm. You can change the "in gear"
rpm for both directions by changing prop size and/or pitch.
This also means you can over-rev the engine by having the wrong prop on the boat. There are engine rpm "stops" for the high end too, but some just rely on using the correct prop. As what you may be experiencing with a "stall" can be brought on by

A word of caution. The inboard/outboard engines can have the same problem as outboard engines. The shift levers are synchronized with a pivot system and adjusting the throttle can interfere with the shifting.

You may be able to view youtube videos by searching for these topics.

Check your throttle body on the intake for carbon buildup. Open the hose to the intake inlet and check that plate that moves when you depress the gas pedal (while not running). Clean debris with damp washcloth, not soaking wet. If a lot of debris the idle air control valve, and chamber will need cleaned.

Engine stalls or RPM fluctuates at idle up and down or dies near idle.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. 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

Possible dirty throttle valve and idle speed control valve.

Throttle Body Cleaning on vehicles without electronic throttle control (most 2003 and down)

You can in most cases repair this slow speed stall or unstable idle by cleaning the engine throttle valve and the idle speed control valve using a product called CRC Spray Throttle valve cleaner. Spray the cleaner into the air intake of the engine with the hose removed from the throttle body, the engine will try to stall so hold up the idle speed manually. Once you have used up about 1/2 can of the cleaner stall the engine out while spraying and turn the key off, disconnect the negative battery cable for 5 minutes and then reconnect and drive the car for a about 25 miles of varied driving modes. If this doesn't cure the problem you may need the fuel injectors professionally cleaned by the dealer service dept.

You vehicle uses a Idle air control motor,(IAC) when you put a load on the vehicle (Drive or reverse) the ECM detects the load and idles up the IAC, even for the A/C (air conditioner) You can try a throttle body cleaning. or a new IAC

you need to change your idle motor (iac valve) this controls the air that engine needs to mix with the fuel to run.simple to do yourself,two screws and one 2 wire plug.

this is your throttle control senson. it needs to be replaced. It should be located on the throttle body assembly (carborator) and will have a wire clip attached to it and will be located right by the throttle cables.

YOU HAVE A BROKEN REED VALVE CAUSES THE ENGINE TO SQUEEL AT HIGHER REVS

Most likely a dirty idle speed control system follow the guidelines below to clean it.

Throttle Body Cleaning on vehicles without electronic throttle control (most 2003 and down)

You can in most cases repair this slow speed stall or unstable idle by cleaning the engine throttle valve and the idle speed control valve using a product called CRC Spray Throttle valve cleaner. Spray the cleaner into the air intake of the engine with the hose removed from the throttle body, the engine will try to stall so hold up the idle speed manually. Once you have used up about 1/2 can of the cleaner stall the engine out while spraying and turn the key off, disconnect the negative battery cable for 5 minutes and then reconnect and drive the car for a about 25 miles of varied driving modes. If this doesn't cure the problem you may need the fuel injectors professionally cleaned by the dealer service dept.