Which of the following could indicate a carburetor issue within an engine?

A popping sound on acceleration is a clear indication of a potential carburetor issue within an engine. This symptom often suggests that there is an incorrect air-fuel mixture, which can result from problems such as a clogged jet, an air leak, or improper adjustment of the carburetor. When there's an improper mix, the engine may experience misfires during acceleration, leading to that distinctive popping sound as unburned fuel ignites in the exhaust system.

On the other hand, an engine that runs smoothly at idle typically suggests that the carburetor is functioning well under low-load conditions, and issues usually become more prominent during acceleration or under load. Reduced cabin noise does not usually correlate with carburetor functionality; instead, it might relate to engine mounts, exhaust system design, or sound insulation features. Similarly, increased exhaust emissions might indicate broader engine management issues, but they are not directly tied specifically to carburetor performance without more context, as they can result from fuel system problems, timing, or other colliding factors. Thus, a popping sound clearly indicates a carburetor-related issue as it points toward an improper combustion process during acceleration.