What are Too Rich and Too Lean mixtures
Too Lean
If the % of CH vapour is too low the mixture will not ignite and is said to be “too lean”.
Too rich
If the % of CH vapour is too high, the corresponding % of oxygen in the air will be too low and the mixture will not support combustion. It is then said to be “too rich”.
Flammable Range
A mixture will only ignite when CH & oxygen are in a proportion range. The contents of CH & oxygen in this range are sufficient to support ignition. This range is called Flammable Range.
In the above Diagram , the pink region is the flammable range.
Ref : Cargo Work by Capt. Errol Fernandes