My T190 had the same problem.
There is three wavy washers that push on the friction disk. Sometimes the washers rust and break. The washers are located in the assembly that includes the throtle handle.
If tightening bolt does not work then:
Lift the cab. Unbolt the assembly and check the washers. Install new if needed, reassemble. You will need to readjust throttle and max speed with the adjustment screws that are part of the throttle lever assembly.
I hope this helps.