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Whenever speed decreases, fuel flow will be stops in MPFI, but not in carburetor. As a result CO & HC emission will be less in exhaust gases.
At starting condition, fuel vaporization does not depends on cranking speed in MPFI like carburetor. So air-fuel ratio can be maintain easily and not being rich. As a result Co & HC will be in less amount.
At starting and deceleration condition, combustion temperature will be low and NOx amount will not be increase.
Air-fuel ratio maintained properly by MPFI system. So it does not forms lean or rich and pollutants will produce less.
Due to the more stratification of charge, exhaust gas produces with comfortable characteristics.

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Comparison of MPFI system & Carburetor in reference to emission

To make pollution less exhaust system, carburetor requires various compensating device while MPFI does not require.
MPFI system inject fuel for A:F ration according to various driving conditions. So that emission decreases.
MPFI system make fuel control through various sensor instead of only manual control. So more fuel does not injects and emission will be in control.
MPFI system changes injection timing of only that cylinder in which detonation occurs. So that emission gets optimum quality.

MPFI system can control fuel supply properly even when ignition fails. So that catalytic converter can not overheat and emission will be in control.
In modern fuel injection system, quantity and quality of fuel are controlled electronically to make accurate control and emission control regulation will be quite.
Exhaust pollution is produce less by MPFI compare to carburetor.