A scooter that doesn't rev or throttle up involves a major failure of the engine and carburettor system. However, the problem frequently has nothing to do with either component. Very often the problem instead involves an inexpensive part. Knowing where to look can help troubleshoot this problem and save a lot of money in unnecessary repairs.
Park the scooter in an "off," non-running position. Remove the body panel covering the scooter carburettor if necessary (it depends on the scooter model). Use a screwdriver to open up and remove the box lid to the carburettor box, exposing the carburettor, if your scooter uses such a container. Use a screwdriver to loosen and remove the headset top from the scooter headset, revealing the scooter throttle assembly.
Use one hand to activate the throttle design by pulling back on the throttle. Watch the throttle cable assembly both in the headset and the carburettor to confirm the cable actually pulls on the carburettor. Disconnect the carburettor if there is no resistance or the cable barely pulls anything, even with a full twist of the throttle handle.
Use screwdrivers and a socket wrench to open up the carburettor body to expose the throttle slide. Check that the throttle cable is connected to the slide inside and pulls the slide up and down correctly. Check that the slide spring has not come loose as well. Close up the carburettor after replacing the throttle cable if necessary with a new one. Test it again to make sure the cable is snug but not tight. Re-install the carburettor.
Remove the body panel covering the scooter engine if necessary. Remove the spark plug ignition cap by hand. Use a socket wrench to unscrew the spark plug in the engine cylinder. Remove the spark plug and put it aside for a moment. Screw in a compression test tool. Stand up and kick-start the engine with the engine kick-start pedal while holding the compression tool dial in one hand.
Repeat the compression test three times after releasing the compression pressure on the tool. Compare the readings with the scooter manual or manufacturer information regarding proper compression on the scooter engine.
Remove the tool, if the readings are correct, and reinstall the spark plug with a socket wrench. Re-install the ignition cap and engine cover when finished.
Pull the engine cover off, if applicable, to gain access to the fuel line going to the carburettor. Use a screwdriver to release the line from the carburettor and use a shop towel to catch any residue fuel dripping out of the line. Pull the line away from the engine and place it over a gas container.
Open the fuel valve of the gas tank and let the fuel flow into the container. Look to see that the fuel is flowing correctly. If not, replace the fuel line with a proper length of 1/4 width new fuel line using a screwdriver. Cut the fuel line to size with good scissors or snippers.
Pull off the old fuel line and attach the new line by hand to the tank and carburettor and secure it with banjo clamps. Do not over-tighten. Make sure to perform the fuel flow test again before attaching to the carburettor.
Replacing the scooter control cables once a year will help avoid old and fraying cable problems that affect not just the throttle but also the brakes and clutch control.
If the problem does not involve the throttle cable, carburettor slide or the fuel line, then the cause of the failure to rev is probably in the engine. Have a scooter dealer work on your engine to solve the problem if you do not know how to rebuild the engine yourself.