Any two stroke outboard... yanking the throttle back from a high rpm to a flat idle would most likely result in having the engine fall on its face. They're just not designed to respond smoothly to that action.
The 1963 40hp Evinrude/Johnson model incorporates a brass fixed high speed jet, located horizontally in the bottom center portion of the float chamber, way in back of the float chamber drain screw. Making sure that the ID of that jet is percectly clean is all that's needed to insure that the fuel flow at high speed is set properly.
Set the adjustable slow speed needle valve as follows.
(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.
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.
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