How to Choose Main Jet
Note: Before tuning carburetor make sure that every filter that is on fuel supply is clean to prevent any false reading on spark plug.
Type and size: there are different type and size of main jet available. but finding out which one is compatible with your carburetor and in the right range is most important.
jet number: the number on the jet does't always mean the size of bore in it. and there are jets that are higher number but are smaller in bore size.
How to choose the right one for tuning a carburetor: first check the stock main jet by riding the bike at wide open throttle for 30 seconds than stop and turn off the bike quickly.pull out the spark plug and read how black or white it is. if it is too black then it means that your bike have too large jet size. however decrease the jet size and ride for 30sec again on wide open throttle and see if it decrease the black color keep decreasing the jet size till you reach the tan brown color.
similarly if the spark plug is white in color. then keep increasing the jet size by 2 number till you reach the tan brown colour.
Bog or Sputter: there is another way to find out if you are too far from the right air fuel mixture is when you are at wide open throttle the bike will either bog or sputter. if it starts to bog or poor performance then it means the jet is too small increase the jet size. or if it starts to sputter at wide throttle and gives poor performance then it means you need to decrease the jet size.