What is a common result of using an excessively lean fuel-air mixture?

Using an excessively lean fuel-air mixture can indeed lead to backfiring. A lean mixture means there is more air in relation to the fuel supplied to the engine, which can result in a higher combustion temperature and misfire conditions. When the fuel ignites too late in the combustion cycle, unburned fuel can be expelled into the exhaust system, where it can ignite, causing a backfire. This typically occurs because the engine is not receiving enough fuel to maintain optimal combustion conditions, leading to incomplete combustion.

In contrast, the other options (increased engine efficiency, smoother engine operation, and better fuel combustion) imply that a lean mixture is beneficial, which is not the case when it becomes excessively lean. While there might be a point of lean mixture that improves fuel efficiency, going beyond that threshold can destabilize engine performance and lead to adverse effects like backfiring. Understanding the balance of fuel and air is vital, as an overly lean mixture disrupts proper combustion and can damage engine components over time.