ex200-2 main pump wont't go to full stroke

Hi... ive been fixing ex2002-2 excavator before. but now i just encounter this kind of problem, when i removed solenoid 1 and 2 to let the pump go to full stroke.. to set the a sensor, i cannot get the correct voltage of the a sensor because the pump will only it will only move until 80% and its like the engine is loaded and black smoke coming out of the exhaust.. i already tried unscrew the unload valve... what could be the reason holding the pump to full stroke?? Please help

ex200-2 main pump wont't go to full stroke