Digital BW, The Print

hi!
i just bought a used samsung ml-4500 (AKA samsung IZZI) B&W laser
printer. it prints rather fine, except when color is merged with black
text - then the black text would print fine, but the color text\image
would print so lightly, almost not visible. this also true when the
original color is very dark, e.g. dark blue or almost black gray text. 
i tried fixing the problem through the printer perefernces, tried
swithing driver, tried through the printer properties->color
management and switching the color profile manualy, but any profile
i've tried didn't fixed the problem. 
i would be extremly glad to find a solution.

thanx,
sharon