The term stoichiometric ratio describes the chemically correct air-fuel ratio necessary to achieve complete combustion of the fuel. This corresponds to a mass Air to Fuel Ratio (AFR) of between 14.6 and 14.7:1. This is known as the "stoichiometric ratio".
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| Spark plug |
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| Carbon fauled spark plug |
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| Normal spark plug |
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| Spark plug (Champion-PRA7HC) of a nearly well tuned Motorcycle |
The color of spark plug must be brown or nearly light black (the exact colour of milk chocolate), the basic rule to know if your bike is properly tuned is that when the color of spark plug is very light black but not very black.
And if the spark plug is black means you got to much fuel (rich mixture) going to engine so you will need to increase the air flow. If its white spark plug (lean mixture) you will need to increase the fuel by tune the air or fuel mixture screw.
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| Air and fuel screw |
Air and Fuel screw, there is one more major difference. Fuel screw turned in (clockwise) gives a lean mixture and turned out (anti-clockwise) gives a rich mixture. Air screw turned in gives a rich mixture and turned out (anti-clockwise) gives a lean mixture. Lean means more air, less fuel. Rich means more fuel, less air.
The default setting for the air screw is one turn out, and unscrewing or turning the air screw counter-clockwise increases the amount of air allowed thru the main jet, after setting at the default 1 turn out, fine tune the each air screw by adjusting one at a time and find where the bike idles highest. Of course this is done after the basic idle is set first.
Many people make the mistake of setting the idle before the bike is fully hot - then find after riding the idle is too high. The idle must be set after the engine is at full operating temperature.
