Carb Problem GZ 125cc

A plugged up main jet in the carb could be the problem. Remove the float bowl and clean the carb. I prefer removing the entire carb from the bike then doing the cleaning. Carb cleaner can mess up the paint job.
Clean or replace the air filter. Another possibility is the muffler could be partially plugged up due to mud or a collapse of the pipe inside the muffler. Disconnect the exhaust then see if the problem is gone. Also, be sure you have clean oil in the gearbox, a well oiled chain, good gas, and a new spark plug.

First check the throttle cable free operation. Check rubber manifold couplings for air leak. If programmable firing control is used check the switch. Refer user guide.
Check program switch operation. This is coupled to the throttle cable. This switch sets the ignition timing advance or ******. This is called power mode operation switch.

Clean the carburetor with diesel or kerosene. Do the cleaning out-doors. Clean all the jet orifices with air. Use correct tools. Especially the flat blade screw driver tip.(end should not be sharp )

Note down pilot jet air screw position. Note the number of turns you screw out. This is needed for resettling the air screw. If it is a CV carburetor check the rubber diaphragm for stiffness or damage.

Needle jet is coupled to this diaphragm. Throttle cable opens the circular gate only. Due to velocity of the air the diaphragm positions the needle. Try the easy checks first. Finally main chain tension. Follow the solution given by Fixya GURU also