Remove the cable from the handgrip and spray some cable lube down the sheath. There is special sprays for this that don't gum up or cause other problems. WD40 , CRC etc, tend to dry out after a short period. I have carefully poured engine oil down my cables and it has kept them sweet for years. Tape a bit of stiff paper in a cone shape around the outer cable sheath and slowly put some oil in. It will slowly seep down the cable. Another way is to fully remove the cable and you can soak the inner cable in a tub of oil. Check that the return springs on the carby have not come off or become loose. Lastly, it could be the butterfly inside the carby. Dirt or damage may be causing the butterfly flap to stick.