What is the reason why the forklift can't reach the speed?

The engine has serious carbon deposition.

The reason why the forklift can't reach the speed is that the engine has serious carbon deposition. Because the throttle valve is too dirty or the fuel injector has too much carbon deposit, there will be serious carbon deposit. Too much carbon deposit will make the ignition system work badly, the spark plug is in poor condition, the oil pressure is unstable and the speed can't be increased.

The engine has serious carbon deposition, so it must be cleaned with professional carbon deposition cleaning agent, or it can be run at high speed to remove carbon deposition.