1990 9.9 Evinrude
From what I've read, you have good compression and good spark on both cylinders... and when pulling the plug wire off the plugs individually, you also receive a equal drop. This indicates good leaf valves, plugs, compression, and ignition. What is left = Fuel problem.
In playing around with the slow speed needle valve, which has nothing to do with the high end rpm of the engine, you state that you can obtain a high rpm. This indicates that you ARE NOT obtaining fuel to the high speed passageways of the carburetor and are substituting instead a fuel flow through the slow speed passageway to obtain the high rpm.
Take that carburetor apart again and look closely at the brass fixed high speed jet which is located in the extreme lower portion of the upper body... in that bulky looking plastic part. If that jet is even slightly clogged, that engine will run exactly as you've explained. When that jet and it's related fuel passageways are cleared, you can adjust the slow speed needle valve as follows, after which you won't need to ever adjust it again.
(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
Note 1: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.
Note 2: If the engine should be a three (3) cylinder engine with three (3) carburetors, start the adjustment sequence with the center carburetor.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
