What's the meaning of stoichiometric ratio and what's the effect of this ration if it's not correct?

The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline fuel, the stoichiometric air–fuel mixture is about 14.7:1 i.e. for every one gram of fuel, 15 grams of air are required. The fuel oxidation reaction is:

Any mixture greater than ~15 to 1 is considered a lean mixture; any less than ~15 to 1 is a rich mixture – given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. These compounds all alter the stoichiometric ratio, with most of the additives pushing the ratio downward (oxygenators bring extra oxygen to the combustion event in liquid form that is released at time of combustions; for MTBE-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles that use an oxygen sensor or other feedback loop to control fuel to air ratio (lambda control), compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition and controlling fuel volume. Vehicles without such controls (such as most motorcycles until recently, and cars predating the mid-1980s) may have difficulties running certain fuel blends (especially winter fuels used in some areas) and may require different jets (or otherwise have the fueling ratios altered) to compensate. Vehicles that use oxygen sensors can monitor the air–fuel ratio with an air–fuel ratio meter.

Air–fuel ratio (AFR) is the mass ratio of air to fuel present in a combustion process such as in an internal combustion engine  or industrial furnace. The AFR is an important measure for anti-pollution and performance-tuning reasons.If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture.

Having a ratio where there is an extra air results in the mixture being lean. The good aspect of running lean is that there is a full air to mix with the gasoline to ensure nearly all of it gets burned however, there is still extra air kicking around that contributes to heat. This results in gasoline savings but the side effect is a very hot combustion chamber which can melt valves and pistons. Another side effect is slightly less power when accelerating.

Having a ratio where there is an extra fuel in the mixture being rich. This results in greater gas consumption but the side effect is a cooler combustion chamber, and inefficient burn as there is not enough air to burn all of the fuel. Another by-product of running rich is that the un-burnt gasoline can ignite in the hot manifold/exhaust and cause them to glow orange. Emissions are also increased and a black smoke can usually be seen coming out the exhaust and it may smell like nail polish.

Stochiometric ratio is the amount of air / fuel mix in the combustion chamber. Incorrect ratio will have an effect in the efficiency of the air / fuel mixture, resulting an incorrect combustion, causing either unburnt fuel being expelled via the exhaust system, or a lean mixture. Both cause loss of power and internal damage if the engine continues to be run for prolonged periods.