What are Lean and Rich as Applied to Nitro RCs?

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Question: What are Lean and Rich as Applied to Nitro RCs?

Nitro or glow engines use nitro fuel but it's actually a mixture of fuel and air that goes into the engine. The right air/fuel mixture keeps the engine running at its best. The wrong mixture can cause overheating and vapor lock, excessive wear, or cause the engine to stall. This fuel/air mixing takes place in the carburetor.

Answer: Lean and rich refer to the mix of fuel and air.

To lean out or richen a nitro RC engine means to adjust the mixture of fuel and air going into the engine. Lean is the addition of more air to the air/fuel mixture. Rich is the addition of more fuel to the air/fuel mixture.


When you lean out a nitro engine you are adjusting the air/fuel mixture so that there is more air going into the nitro engine than there is fuel. This provides a little more horsepower but can result in very high engine temperatures. If you are not careful leaning out a nitro engine you could run it too lean. This will wear out the glow plug prematurely or cause engine failure.


When you richen the nitro engine's mixture you're adding more fuel than air to the nitro engine. This can give you better results for some kinds of races because this method, unlike leaning out, will give you cooler engine temperatures. But if running too rich you can not only bog the engine down and stall out but also flood it and foul the glow plug.

When to Lean Out or Richen a Nitro RC

You might be running too lean if the engine dies while idling, you don't see a light stream of blue smoke from the exhaust, or the engine gets so hot that a drop of water on the engine immediately starts sizzling and popping.

Too much blue smoke or a lot of unburned fuel from the exhaust and an inability to reach top speed are some signs that you may be running too rich.

How to Lean Out or Richen a Nitro RC

Engine tuning and adjusting the air/fuel mixture involves adjusting the high-end (high speed / engine temperature) and low-end (low speed / idle speed) needles on the carburetor. This is also called dialing in your engine. There are usually baseline settings for each nitro engine that provide a good starting point for adjusting the needle settings. You'll turn each needle in very small increments to lean out or richen the fuel.

Turn clockwise to lean out or add air and counterclockwise to richen or add fuel. The low-end needle controls idling and low speeds. The high-end needle controls how the engine accelerates and runs at high speed and has a greater effect on engine temperature. See closeup illustration of fuel/air mixture needles.

Lean, Rich, and Engine Temperature

You want to adjust the air/fuel mixture so that your engine runs at an optimal temperature which is generally somewhere between 225-250 degrees Fahrenheit for most nitro engines. Much over 250 degrees could cause a lot of damage and also shortens the life of your nitro engine.

Check your nitro engine's temperature often to keep it at an optimal temperature for longer run times and overall better life for your nitro engine.

If the running temperature is less than 200 degrees you need to turn your high-end needle adjustment clockwise to lean out the mixture a bit to get the temperature up a little. If your temperature is above 250 degrees you would bring it down by adjusting the high-end needle to richen the mixture by rotating the high-end needle counter-clockwise. The ambient temperature outside and the elevation according to sea level will adversely affect the nitro engine's temperature so adjust accordingly.

For more detailed instructions on engine tuning, engine temperature adjustments, and leaning and richening needle settings see these tutorials: