Phosphate bonded investment should be completely carbon free, otherwise:
Investment chips off when molten metal enters the mold space
Black castings
Carbon makes the casting brittle
Smooth casting
Carbon is often added to the powder to produce clean castings and facilitate the divesting of the casting from the investment mold. This addition is appropriate when the casting alloy is gold, but there is disagreement regarding the effects of carbon in phosphate investments used for casting silver-palladium alloys, palladium-silver alloys, or base metal alloys. It is believed that carbon can embrittle the alloys, even though the investment is heated to temperatures that burn out the carbon. Some evidence indicates that palladium reacts with carbon at temperatures above 1504 °C. Thus, if the casting temperature of an alloy with high palladium content exceeds this critical point, a phosphate investment without carbon should be used.
Ref: Phillip’s 12th edition page 208