Make sure the scooter is warmed up and at normal operating temperature (5 mile ride).
When the mixture is slightly richened, the idle will speed up, almost imperceptibly. In fact, that is how to adjust it properly.
Put a reference mark on the carb body to line up the mixture screw (In the US the mixture screw requires a special tool, in Europe it is a normal slotted screw,commonly referred to as the pilot jet). Here is a picture of the correct screw to adjust. In Europe it does not require a special tool, just a slotted screwdriver.
Turn the mixture screw clockwise, until it snugs up against the carb body, counting the number of turns to fully in. Then wind it back out to the original position.
Start the scoot and turn up the idle a bit so that it idles fast. Now turn in the mixture screw until the engine begins to falter.
This is leaning out the mixture. Now back the screw out a 1/4 turn at a time, leaving a mintue or so between 1/4 turns until you have the fastest possible idle. This is somewhere between 2 and 3 1/2 turns out. Then readjust the idle.
When you get to fast idle you should fine adjust using 1/12 of a turn. I use one of these as you can see the rpm alter for an almost imperceptible movement of the screw. You should be able to hear it too though.
When you weaken the mixture, the idle will slow. If you go too rich it will slow again. Somewhere in the middle it will be at its fastest.
The Draper tachometer (not necessary - use your ears or any other tachometer that operates from clamping on the plug lead. Some multimeters have this facility):
You'll need this tool in the US for mixture adjustment, which costs less than $20 from Motorsport Scooters: