Difference Between Rich and Lean Mix in Fuel System

Difference Between Rich and Lean Mix in Fuel System – Maybe in our daily life, we often hear that “Hey, your engine seems to be too hot, try adjusting the carburetor so there won’t be a lean mixture”. Or there is also “if when we want to warm up the engine in the morning, the carburetor valve must be closed so that a grease mixture occurs”.

Difference Between Rich and Lean Mix in Fuel System

Rich and Lean Mix in Fuel System
Rich and Lean Mix in Fuel System.

The conversation above is a fact that is often heard, but not everyone knows about its meaning. Especially if you are a mechanic or someone who likes to tinker with your vehicle. You don’t have to fully know the theory, but if you know about this theory, it will be easier for you to get the results of your machine settings as you expect.

The theory of the mixture of fat and lean is closely related to the theory of mixing air and fuel.

The ideal mixture of air and fuel that must occur in a combustion engine is divided into 2 types, namely units of weight and volume,

In units of weight ( 15: 1 )
In units of volume ( 9000: 1 )

Where in one liter the weight of fuel and air is different,
1 liter of air = 1.3 grams
1 liter of fuel = 780 grams

What does this have to do with the mixture of fat and skinny with the data above? We will get a mixture of fat and skinny by comparing the current (actual) mixture with the ideal mixture.



We use the example in units of weight
1. Actual A/F is 5: 1
2. Actual A/F is 30: 1
What mixture occurs in numbers 1 and 2?


  1. 15: 1/5: 1 = 3 (The value of 3 is greater than 1, the conclusion is “fat mixture)\
  2. 15: 1/30: 1 = 0.5 (The value of 0.5 is less than 1, the conclusion is ‘thin mix)

Note: The value of 1 is taken from the Lambda calculation value.

If you are still confused by the numbers above, then there is a very simple analogy about this.

Standard value
– 1 cake pan ration for 15 people. The cake is cut into 15 pieces and distributed to each person 1 piece.

First incident
– 1 baking sheet ration for 15 people. But only 5 people came. So everyone gets 3 pieces. (which should be 1 person 1 piece, gets 3 pieces. Exceeding the standard, later people will get fat quickly)

Second incident
– 1 cake pan rations 15 people. but who came very much 30 people. In order for everyone to get a share of all, then 1 piece of cake is divided into 2 people. (What should have been 1 person got 1 piece, this actually got half a piece of cake. Less than the standard of eating. All the people will be skinny).

Maybe it’s a little explanation about the mix of lean and mixed rich user data and simple logic. May be useful.

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