Question

Why isn't C a product of the incomplete combustion of acetylene, even though during a lab there is soot on the test tube?

Answer 1

Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide. As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over.
The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke. Carbon monoxide is produced as a colorless poisonous gas.