As the slide is lifted any reduction in vaccum, thus a leaner mixture, caused by a worn slide is reduced. The symptoms of a worn slide are more prominent at lower throttle openings.
So as the slide is raised air leaking around the slide has less and less effect upon the operation of the carburetor.
You don't tell us where the throttle is when you are having these problems. In its simplest form the AMAL
works in stages, or jetting circuits. When describing symptoms it is important to relate it to the slide position. This tells us which circuit, or circuits as there is some overlap, is causing the problem.
It is possible to compensate, by changing the circuits involved, to compensate for a worn slide. When you reduce the signal on the jet it is a simple matter of increasing its size. This is exactly what AMAL
did when they increased the clearance on the slide to prevent the slide from sticking when the carb body was over tightened.
What a lot of people aren't aware of is, there is a natural compensation for the effects of a worn slide. As the slide wears so does the needle jet. As changes in the diameter of the needle effect the carburation at the same point or circuit were in the slide does, the leaking air around the slide is compensated by the increase in gas flow through the worn needle jet.
When we really start to see a lean problem is when we replace the needle jet and we no longer have the extra gas to compensate for the air leaking around the slide lowering the vaccum on the jet.
The symptoms you offer can be caused by any one of the jetting circuits, or a combination. Thus the bike starting to accelerate a bit as you start to close the throttle above 2/3 - 3/4 throttle could be a sign of a too small main jet or needle set in a too lean position. The same symptoms at 1/4 throttle call for attention to the size of the needle jet and its condition, fuel level in the bowl, a modified cross drilled hole larger than .035", a blocked pilot jet, or air adustemnent screw in wrong position or slide cutaway.