Final answer:
The least likely cause of one cylinder contributing less power in a carbureted engine would be a faulty carburetor since it typically affects all cylinders, unlike other localized issues.
Step-by-step explanation:
When a cylinder balance test indicates one cylinder is contributing less power in a carbureted engine, we must evaluate common causes. A faulty ignition system could lead to poor spark quality, thus incomplete combustion. Similarly, a burned exhaust valve can cause compression loss and subsequently poor cylinder performance.
A leaking intake manifold would also disrupt the air-fuel mixture's entry into the engine, leading to power loss. However, a faulty carburetor tends to affect all cylinders, not just one. Given this, the least likely cause among the options is a faulty carburetor since it would usually cause a more systemic issue rather than an isolated cylinder problem.