Disconnect from a branch pipe 2 the valve a hose 4 going from the air filter.
Start the engine and bring the engine speed to 3000 rpm, then release the accelerator pedal. Throttle stop screw (shown in the photo with an arrow) should return to the position of falling engine speed after a short period of time, i.e. get stuck in. At the same time, a change in air pressure should be felt in nozzle 2.
If the air pressure does not change, check the tightness of the hoses and their connection to the carburetor 3. If the hoses are intact, replace the air valve 1 of the engine braking system.