3.5 mercury outboard stalls when put into forward

So with the throttle closed, held closed by hand, does it still over-revs as soon as you start it up? If that's the case, disconnect the top spark plug wire and restart it, if it doesn't over rev, the issue is the upper crankshaft seal. This is a fairly difficult job so I would suggest seeking out a marine mechanic unless you are very mechanically inclined.

what you are trying to say is that the engine wont rev up? or it revs up fine with no load but cant rev up when trying to push the boat? maybe its still running on 5 cylinders

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.

That is best. A very short pause is fine.

I’m happy to assist further over the phone at https://www.6ya.com/expert/bill_f6ca18ca1db028ed

If it stalls out when putting it forward or reverse, then the problem could be a jammed gear train. If it still runs when shifted into gear but doesn't spin the prop, then the prop shear pin may have broken. This is done to prevent engine damage if the prop hits something hard such as an underwater log or such.

An engine requires much less fuel to rev in neutral, with no load on it compared to an engine in gear pushing a vehicle.
The first item I would check is the filter screen on the side of the engine. Take off the cowling and look for the fuel hose as it flows in from the gas tank under the cover. It will terminate at a plastic fitting with a threaded screw in the center. That is the fuel pump. After removing that screw there is a fine mesh screen which is the fuel filter. Make sure that is squeaky clean. Also check for any other inline filters and replace if they haven't been in a long time. last but not least, make sure the vent on the tank is open/not blocked.

have the carby overhauled and cleaned out
check all rubber components for failure inside hoses and fuel pump

It's possible that a little fuel vapour come out of a carb on an hot engine that has just stopped.
Does the motor starts again and runs well in higher rpm ranges? Is the stall only when idling? If so, enrich the idle fuel mix ( screw out the idle mix screws half a turn t a time )

Hi, did you buy your engine used? Sounds like your gonna have to get the carb or carbs cleaned. Its possible you could have a stopped up fuel filter, or not enough spark but i am guessing its gonna be the carbs. They got some gunk in em i bet. Probably old fuel turned to gunk from setting up. I had the same problem on my 40 horse.