When does a carburetor economizer system needle valve begin to open?

• A. At idle speed
• B. At maximum power
• C. At 60-70% of rated power
• D. At 30% of rated power

Answer: (C)

The main idea is that the economizer helps save fuel by leaning the mixture as engine load rises. The needle valve in the economizer is designed to start opening as you move into cruise or moderate-to-high power, so you get more air relative to fuel and a leaner, more efficient mixture. This typically happens around 60-70% of the engine’s rated power, which is when you’re past idle and light throttle but not at full power. Opening at this point keeps the engine running smoothly and efficiently for steady or cruising operation.

If it opened too early, at idle or very light load, the mixture would become too lean for stable idle and acceleration. If it opened only at maximum power, you’d miss the fuel savings during common operating ranges. Opening around 60-70% provides the best balance for economy during normal cruising, which is why that option is the best choice.