Why balance motorcycle carburetors

Instead of using a bypass "air path", a small bypass "oil path" is designed on the outside of the carburettor to adjust the mixture ratio by using the oil feed from the bypass. 

The mix ratio screw is designed into the bypass and the amount of fuel fed into the bypass is controlled by turning the mix ratio screw in and out, which also controls the mix ratio. When the mix ratio is adjusted in this way, the mix ratio screw is adjusted in the opposite way: when the mix ratio screw is turned "in" clockwise, the passage in the bypass becomes narrower, less oil is fed and the oil and gas becomes thinner (higher A/F); when the mix ratio screw is turned "out" counterclockwise, the passage in the bypass becomes wider, more oil is fed and the oil and gas becomes thicker (lower A/F). This will result in a wider bypass passage, more oil being fed and the oil and gas becoming thicker (lower A/F).

The common way of adjusting the mix ratio is to turn the mix ratio screw gently clockwise to the bottom and then turn it one and a half turns counterclockwise to get the right mix ratio position. For finer adjustments, adjust the idle speed to around 1300 rpm and then fine-tune the mix ratio screw from side to side, adjusting it to a point where the idle speed will rise or fall. Then lower the idle speed back to the speed you think is appropriate, then turn the mix ratio screw again and the speed will drop regardless of whether it is adjusted counterclockwise or clockwise, then it is the best mix ratio.

Alternatively, you can look at the spark plug colour and adjust it. If the mix ratio screw is messed up, turn it back to the one and a half turn position, then clean the spark plugs, ride the car for a while (or start it and let it idle for half an hour), and then check the spark plug colour. If the colour is too white, make it stronger, if it's too dark, make it lighter. The main disadvantage of using this method is that you can't use too much throttle opening when you "burn the spark plugs", otherwise the spark plug colour may not be accurate.

In any case, the above method of adjustment is not a precise one. In order to make a serious adjustment, it is necessary to use an instrument to measure the exhaust gas, and use the measured value to see how the gas and oil are burning, and to adjust it according to the throttle response. Therefore, it is better not to mess with the mixture ratio unless the car is in really bad condition, or the spark plugs are obviously not the right colour, or the engine or carburettor has been modified and must be adjusted.