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How to read Spark plug

spark plugs colour and condition tells a lot about what going on inside the combustion camber

such as air to fuel ratio and what is the condition of piston rings and valve seals

air fuel ratio: there are three types of air fuel ratio 1.rich 2.stoich 3.lean

stoich: for a gasoline or petrol engine air fuel ratio 14.7:1 is consider stoich. it have 14.7 parts of air for every 1 part of fuel and when it is burned inside the engine all the fuel is burned along with all the air and no excess air left.

we can tell that if the air fuel ratio is stoich. the spark plug have light brown or light gray colour on it.

spark plug running stoich air fuel mixture

spark plug brown tip and  ceramic insulator brown color indicates that the mixture is stoich

rich:  air fuel ratio is considered rich if it is  12:1 at this air fuel ratio spark plug colour is little black. but not too much black like show in the picture below. a spark plug with too much black colour will consider a carbon foul spark plug. when this happens you will need to lower the fuel ratio if the problem still persists then try changing spark plug.

carbon fouled spark plug

spark plug Black tip indicates that the mixture is too rich or carbon fouled

Lean:  air fuel ratio is considered lean. if it is  17.6:1 at this air fuel ratio spark plug colour is slightly white and all the fuel inside combustion camber will completely burn. but excess air will left. fuel creates a cooling effect inside the combustion chamber and the right amount keeps the air fuel ratio stable . anything above 17.6:1 will consider unstable and cause knocking and detonation. it is recommended to not go for lean air fuel ratio with carburetors. only run lean with E.F.I systems. because they are precise at delivering fuel.

Poor Fuel Quality : A poor quality fuel also cause spark plug to run hot which gives white colour at spark plug.

hot running spark plug

spark plug white tip indicates that the  mixture is running hot

Broken valve seals and worn piston rings : if piston rings are worn or valve seals are leaking then spark plugs will have oil residue. inspect both of them and replace the worn part

How to test Each Circuit

this is the most important part of adjusting the carburetor. to tune a carburetor, you have to know how to read a spark plug. because the spark plug tells how the engine is running. is it running rich or lean?

first, start the engine. warm it up and then take on a ride. after 20min of a ride come back and let the engine cool for about 5min. then remove the spark plug this will show you the overall condition of the air-fuel ratio. if it is black then it means the engine is running rich. if it is white then it means the engine is running lean and if it is rusty then it means the air-fuel ratio is stoich in simple words the carburetor is running fine.  but it is the overall condition there could be some points where the carburetor is too rich or too lean.

How to fine-tune the adjustment by reading Spark plug:  

1: stick a piece of tape to the throttle body. stick another piece of paper to the throttle and mark the point where there is no Freeplay on the throttle. indicating idle or zero throttle position. now completely twist the throttle and mark the point where the throttle is fully open. mark it as 100% throttle. then mark the middle point 50% and for the middle point between zero to 50% write 25% and for the point between 50% to 100% throttle position write 75%.

Pilot jet or idle: start the engine first and ride the motorcycle below 25% throttle. then stop and pull out the plug and read. if spark plug is black or white then it means idle needs adjustment.

Needle jet: start your bike warm it up and ride between 25% to 75% throttle position for a few minutes then stop and pull out the spark plug. if the air-fuel ratio is off it means needle height needs to be adjusted.

main jet: ride between 75% to 100% throttle position. if the air-fuel ration needs the adjustment then change the main jet size to adjust that point.

throttle position at 0%

throttle is at position or 0%

throttle position at 75%

fully open throttle or 100% throttle position

spark plug showing lean air fuel mixture

spark plug white tip indicates that the  mixture is lean

carbon fouled spark plug example

spark plug Black tip indicates that the mixture is too rich or carbon fouled

spark plug showing stoich air fuel mixture example

spark plug brown tip and  ceramic insulator brown color indicates that the mixture is stoich

Learn more about carburetor tuning in following articles

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