You can access your carburetor for adjustment by opening up your trunk and removing the screw that is in the bottom plate The Triton R4 is pictured to the right, on the Zip R3i the carburetor should be visible underneath the back seat as shown below.




In this picture you can see the idle adjustment screw, the mix screw, and the throttle adjustment sleeve. You can adjust the idle screw with your fingers, but the mix screw requires a short flathead screwdriver.




The mix screw shouldn�t be adjusted unless you are having problems with it starting, hesitation, stalling, etc. It would be a very good idea to mark on the carburetor what position the screw was in initially (if it didn�t come like that already) and write any modifications you make to the mix screw down so you know how to fix it back the way it was if you mess up (it ships from the factory at 1.5 turns out). The mix setting is altitude specific, so there isn�t a setting that works universally. You have to see what works best for your situation. The fuel/air mix screw is for idle conditions only, the fuel/air mix for wide open throttle is a adjusted by replacing jets in the carburetor. If the factory settings aren�t working for you, adjust it a half turn at a time, and give the scooter a trial run (10-15 minute drive (after a proper warm up!)) to see if the problem you were experiencing is resolved.



Adjusting the Throttle
This can be done in two places, the first place is on the carburetor itself, the second is on the handlebar, just below the throttle grip. By making this adjustment, you can get better throttle response, and possibly more powerful takeoff and higher top speeds. It isn�t as easy as adjusting the idle or the mix though. The first thing to do is get to your carburetor, and the rubber boots that are on the throttle adjustment shown in the picture above need to be slid back so you see something like this picture:


Now that you are unobstructed by the protective rubber, turn your idle down (counter clockwise) until the scooter is just barely running and is on the verge of stalling. Now loosen the lock nut (clockwise) and then move the adjustment sleeve out further (counter clockwise) until you notice that the idle on the scooter has changed. Move it back down so that this adjustment doesn�t affect your idle (make it so that it�s about to stall again, but the adjustment sleeve is out as far as it will go without making the scooter rev or idle faster). It may help to hit the throttle a few times, just like when you�re adjusting the idle, to make sure that the sleeve isn�t out too far. Once you have it at the proper position, tighten the lock nut down (counter clockwise) with your fingers, and tighten with an adjustable wrench so it�s not going anywhere.

You will now need to adjust the throttle cable in one more place - on the handlebars.


It�s the same thing here as on the carburetor. Loosen the lock nut, bring the sleeve out as far as it will go without affecting your idle, then tighten the lock nut back down.

The last step is to readjust, or make the initial adjustment your idle as described earlier on this page



Draining the Carburetor
Draining your carburetor every now and then is helpful to remove debris (like dirt and water) that are trapped in the carburetor bowl. This can also be an effective way to cure a stuck float (which can sometimes be remedied by lightly tapping the bowl shown to the right with a soft object like wood or the plastic handle of a screwdriver). In the picture I have indicated the air intake connected to one side of the carburetor, the carburetor bowl, the drain screw, and the drain hose. The drain hose is the hose coming off the bottom of the carburetor that is not connected to anything else (mine was hanging loose under the scoot from the factory, I tuck mine in the same bracket that holds the brake line to the bottom of the transmission casing for aesthetic reasons - this would be different on the Zip R3i as far as location, look at the first picture on this page, you can see the hose coming off the bottom of the carburetor).
You can drain the carburetor on the ground, but I prefer to put the unconnected end of the drain hose in a glass or clear plastic cup so I can see what actually drains out (water collects at the bottom, usually along with any debris that have found their way into the system - don't use styrofoam because gas will eat right thru it). Either way you want to do it, just turn the drain screw counter clockwise several turns, or until the fuel is draining at a fairly steady rate, then wait till the flow of fuel stops. Then tighten the drain screw back down (clockwise) to put everything back the way it was.

Phazei adds:

Note about the idle:

You can choose to idle as low as you can make it while it still stays alive. Though if you're having battery problems, where the battery isn't getting charged like it should, it is possible it is because of the idle. When you are at a stop light if your scooter isn't idleing high enough to cause the stator to generate enough power to run your lights, it drains the battery. I usually idle from 1500-2500.
To know your max idle speed, put the bike on it's center stand and keep turning the idle up. When the back tire starts to move, you've gone too far. Go back a turn or so and that would be a good max idle. Remember, that can also waste gas.

Note about the fuel/air mix:

Turning it CLOCKWISE makes the bike LEANER. Turning it COUNTERCLOCKWISE makes the bike RICHER.

To find a good spot for it, turn it in either direction 1/4 turn at a time. Continue to turn it in the direction that causes your idle speed to go up. Once the RPMs stop going up, turn it clockwise 1/4 turn from it's highest RPM position, that'll make sure you're a tad rich, which is good. Keep it there and then readjust your idle to whatever you want it to be. 

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