AG Mechanics CDE Practice Exam Prep – Practice Test & Study Guide

When black smoke is coming from the exhaust while the engine is running at 3000 rpm, the most probable cause is a rich high-speed air-fuel mixture, which is indicated by an excessive amount of fuel in relation to the amount of air in the combustion process. This can happen due to issues such as a malfunctioning carburetor or fuel injection system, causing an overabundance of fuel to be burned.

The other options can be ruled out:

- A dirty air filter (Option A) would typically result in a restriction of air flow, leading to a lean air-fuel mixture rather than a rich one.

- A malfunctioning spark plug (Option C) could cause misfires or incomplete combustion, but would not directly result in a rich air-fuel mixture producing black smoke.

- A low fuel level (Option D) would not cause black smoke, as the issue lies in the ratio of fuel to air being too high, rather than the absolute quantity of fuel present.