My 40HP Mercury outboard motor revs at very high rpm when in neutral. When advancing or reversing it returns to normal but when placed back in neutral it revs again. Any ideas as to the problem?

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.

if ignition attached to a dashboard fuses in behind dash board. should be marked. if motor is tiller steering then the fuses are inline or tucked in housing. back track from problem.

Hey Barbara,, get one of these manuals,,, you will need it to trouble shoot this outboard..

Mercury Outboard Manuals by Seloc 1965 2014

propellor pitch deopends on the work required by the operator and is sized to mainytain the torque rpms of the engine
too course and the engine labors and too fine and it over revs
best talk with a mercury service shop and tell them exactly what you are using the boat for ( low speed trolling , fishing etc , ski-ing
high speed to get there and back) they will advise on the correct pitch to suit the requirements that suit the engine

Might be fuel problem. Did you try changing the gas and getting it from a different station?

I have a problem with my 2 stroke 20hp mercury 1989,when I run 10-90% throttle it's works fine but when in 100% its only runs in 90% and doesn't sound normal like it's not burning right.

compressions more than 100psi ? carb clean and fuel not older than 5mths ? timing correct ? running on all cylinders ?

poss carbs need cleaning as less load in reverse ? crank seals could be leaking ! compressions more than 100psi ?

you could try a lower pitched prop.2" would up the revs by 400 revs.,which would up the revs ,but give you more low down thrust,and/or tucking the motor under more[if you have electric trim/tilt].thats a big boat and fair bit of weight for a 40hp..my 40hp merc is on a 13ft saracen,and with two of us it takes a while to get gooing.so i bought a ProPulse prop,with adjustable pitch.been god to try different pitches

hi take the prop off see if there is anything /string/ between the hub and the housing check oil lvel in g/box /is gear box nice and free if all is ok try it in ware with just a small amount of choke
drain /empty fuel out replace with fresh /correct oil mix
check /change fuel filter/air filter
warm engine up check idle mixture ajustment/tick over speed
increase idle speed slightly/7/800 rpm
hope this helps merlin v 12