I have a 150cc GY6 Ruckus look-a-like scooter. A couple months into owning it the sending unit stopped working. After trying a new gauge I figured out that it was the sending unit not giving me the proper information. This is how I fixed it.
Philips Head and Flat head Screw Drivers
Socket Set (my bolts had a 10mm head on them)
A well ventilated area to work
A clean catch basin to collect the draining gas
fuel filter (optional)
Nitrile Gloves (optional)
First thing I need to get to the gas tank. My tank is located under my floor board and it is held on with 4 bolts and 2 screws. Your fuel tank location may vary. Once the floor board is loose the tank is now free fromt the frame. Unhook the wiring connector fro the sender. Also have a catch basin ready when you disconnect the fuel line to catch the gasoline.
Step 1: Removal of the Sender
Use a flat head screwdriver to push on the tabs to turn the metal compressor on top of the sender. Once loose slip it off the wires, and gently lift until the unit is out of the tank. you will have to do a bit of wrangling since the arm and float are at an angle. Don't loose the rubber gasket underneath.
Now its out time to test and fix.
Step 2: The Trouble Part
Notice in the picture when you push the float arm up the fuel gauge shows full.
When you push the arm down it should show full as in picture two.
The arm should fall down when not held up. Mine did not.
In picture three you can see my two fingers gently pulling on the arm towards my palm, away from the unit. Don't over do this you want to looses it up not ripe it apart. Re test, lift the arm up and see if it falls freely if it does the sending unit is now fixed. Test again to make sure that when you lift it up the gauge shows full and when it falls it shows empty.
Step 3: Reinstall
Time to reassemble: put the rubber gasket back on the opening, then gently put the sender back in alining the tab.
Next add the compressing nut back and tighten with the flat head screwdriver.
Place the tank back into the frame and reconnect the wires.
If you don't already have one, add an inline fuel filter before your fuel pump.
Now bolt the floorboard back onto the frame and refill the gas tank.
Now Go ride safely.