posted 05-14-2014 02:29 AM ET (US)            

I think you might have the type of 90-HP FOURSTROKE that uses a power head made by another outboard engine manufacturer, which Mercury bought from them, and then built into their own engine with different paint and decals. Your mention several times of "an idle screw" and wanting to "open it up" makes me think you might have the type of 90-HP FOURSTROKE that has carburetors.

First of all, you should be aware that you are not supposed to be making adjustments to the carburetors because that engine was sold under EPA compliance with exhaust gas emissions. Owners are not supposed to tamper with any part of the engine that could alter its exhaust gas emissions. I just want to tell you that--you do not have to pay any attention to me.

You should pay attention to this: the Mercury 90-HP FOURSTROKE that has carburetors and is actually not a Mercury engine at all, but a repainted engine made by a competitor of Mercury, is somewhat famous for having problems with the carburetors, particularly the low speed or idle speed carburetor jets. It seems those jets become clogged or blocked or varnished or otherwise affected by routine operation and result in poor idle characteristics. There are many prior discussions of these problems.

I don't own one of these engines, but, as I recall, from reading the many reports about them, and from getting emails from people who owned them, the remedies range like this:

--take it back to Mercury and trade it in on a newer model. (It is probably too late for you to follow this advice, since it sounds like you are not the original buyer of this engine from Mercury);

--try doping your gasoline with a strong concentration of solvents or cleaners for gasoline fuel, and run the engine for a while on this home-brew gasoline, all the while hoping that it clears out the muck or debris or varnish or whatever it is that is causing the problem with the carburetors; or,

--disassemble the carburetors and carefully clean them, paying special attention to the low speed orifice or jets, and, with great skill, put those carburetors back together, reinstall on the engine, reset all the linkages, and get the engine running again; repeat this as often as needed; or pay a mechanic to do this.

At the very least you should probably get some Mercury QuicKleen treatment for the gasoline and mix it in a strong proportion following the advice on the label with your gasoline fuel.

It is common for the idle speed of an outboard engine to change when it is run without being immersed in water. The exhaust path on an outboard engine is through the propeller. If the propeller is not in water, there will be much less back pressure on the exhaust, and this tends to affect the idle speed characteristics. It is not a good idea to make any really touchy adjustments on an outboard engine when it is not in the water and its normal operating environment. Running "on land" as you say, with a hose adaptor, will change the behavior, particularly at idle speed.

posted 05-14-2014 09:16 AM ET (US)            

These engines need to be run in order to keep the carbs clean.
With real fuel, not garbage alcohol, they would be better.

So, as stated, clean out the tanks, pick up filter in tank, hoses, fuel filter(s), carbs, and balance the carbs. Quicklean is a good product.
Idle should be around 800. Or it will be difficult to shift. (Possibly the cause of the shift shaft problem)

Many posts on this site, as mentioned.
Search here and other sources.

posted 05-14-2014 09:16 AM ET (US)            

As for only seeing the smoke when operating the engine on the water, this could, again, be an artifact of the through the propeller exhaust path being underwater when in the water and open to the air when on the trailer. At idle speeds on an outboard engine when in the water, there is too much back pressure on the through the propeller path. There is usually an exhaust bypass path that is open to the air. The engine exhaust goes out the exhaust bypass path at idle speeds. Exhaust does not go out through the propeller until the boat begins to move forward as some speed. The exhaust bypass is usually somewhere on the aft edge of the midsection of the engine.

posted 05-14-2014 10:41 AM ET (US)            

Steam would indicate an overheating condition. You said the shift shaft was just replaced. That would involve the removal of the lower unit and I don't like coincidences.