Common fault of carburetor: idle instability

 Idle speed instability

 Idle instability phenomenon: after the engine operation for several minutes, the engine idle speed fluctuation is more than ± 100 RPM is idle instability.

The cause of idle instability: when the oil or airway of the oil channel of the carburetor idle system is blocked or leaked, the oil supply of the idle oil system is sparse or concentrated, resulting in the idle instability of the engine.


1.The idle volume hole is partially blocked

 Reason: the idle volume hole is partially blocked, so that the oil supply is sparse at the idle speed, resulting in the idle instability phenomenon.

Exclusion method: clean according to the above carburetor cleaning method. 


2. Idle adjustment screw (commonly known as "wind needle") position changes

The function of the idle adjustment screw is to change the circulation section of the idle oil channel or airway by adjusting the idle adjustment screw, so that the idle oil supply of the carburetor reaches the ideal state. The idle adjustment screw is divided into two types: oil regulating (such as CG125 carburetor) and gas regulating (such as Mulan 50 carburetor).

For the professional carburetor manufacturer: because the idle adjustment screw has a great impact on the performance of the engine, the carburetor has been strictly tested and the idle adjustment screw has been adjusted to the best position before leaving the factory. Therefore, users are generally forbidden to adjust the idle adjustment screws. After a long period of use, if the idle adjustment screw position does change and cause adverse consequences. There are two ways to find the best position for the idle adjustment screw:

(1). The best adjustment method

First fix the plunger to bi often idle product speed slightly higher engine speed, around rotating idle adjustment screw, find out the highest speed of the plunger position, slightly adjust the plunger adjust the screw, reduce the engine speed to find the highest speed, so repeated, until a plunger position when the highest speed is equal to the vehicle standard idle speed. For four-stroke engines, sometimes the concentration value of CO is still very high after the best adjustment, so the Paris adjustment method can be appropriately adopted.

(2) The Paris Adjustment Law

 The Paris adjustment method is based on the basis of the best adjustment method, it intentionally rotates the idle adjustment screw to the direction of the mixture thinning (at most 1 / 8 rotation), then the speed should be reduced, and then adjust the plunger to restore it to the original speed. The result of the adjustment should make the HC value slightly rise and the CO value decrease. The principle is that HC cannot rise too much, and CO is slightly lower than the standard. If the result of the Paris adjustment method makes the CO reach the standard, and the HC exceeds the standard is not allowed. If both CO and HC cannot meet the standards, it means that the carburetor cannot meet the emission requirements when the conditions do not change. It can also be seen here that limiting CO and HC can ensure the rationality of the adjustment. Otherwise, the CO is blindly lowered, so that the HC is too high and the combustion is in a very unreasonable state.

If the best adjustment method is used to meet the emission standards, it is best not to use the Paris adjustment method. If the HC and CO exceed the standard, the Paris adjustment method can be appropriately used properly. If the Paris adjustment method cannot make CO and HC meet the standards, the carburetor and ignition system should be checked.


3.The carburetor and the engine inlet pipe connection gasket or rubber are damaged

 If the connection gasket or rubber ring is damaged, it will leak air. Additional air enters the engine to dilute the oil supply at idle state, resulting in idle instability.

Exclusion method: replace the connection gasket or rubber.


4.The connecting bolt between the carburetor and the engine intake pipe is loose

 The loosening of the connection bolts will also cause air leakage phenomenon. Exclusion method: you can tighten it down.

One thing to be pointed out is that most of the carbureters used in scooters are charged thermal bypass thickening system. Under the action of this system, the idle speed of the motorcycle is higher (up to 2200~2300 RPM). After the heating engine 4~5 minutes electric bypass thickening system is closed, the engine idle speed drops to 1500 RPM. This is a normal phenomenon, and it is not a "idle instability" fault. Hope the user to be careful not to misjudge.

Common fault of carburetor: idle instability