What was found
DURING DESCENT FROM 25K FT TO 23K FT WITH THE POWER BACK TO IDLE & NAC HEAT AND WINDSHIELD HEAT ON. NOTICED A SPIKE IN CABIN CLIMB RATE AND LOOKED AT GAUGES TO DETERMINE THE EMERGENCY BLEED VALVES HAD OPENED AND THE CABIN ALTITUDE WAS AT 8000 FT WHEN IT HAD BEEN SET FOR A FIELD ELEVATION OF APPROXIMATELY 6000 FT. AS IS NORMAL, THE EMERGENCY BLEEDS PUSHED UNFILTERED SMOKY AIR INTO THE CABIN. DECLARED AN EMERGENCY, A NORMAL LANDING WAS MADE WITH NO FURTHER INCIDENT. TROUBLESHOT THE PRESSURIZATION SYSTEM AND FOUND THE RT BLEED MIXING VALVE POPPET NOT COMPRESSING, AND THE LT BLEED MIXING VALVE POPPET ONLY COMPRESSING HALFWAY. BOTH MIXING VALVES WERE REPLACED, SYS CHECKED OUT AND NOW OPERATES AS NORMAL.