if your diesel engine / diesel generator speed is unstable, you need to check the reasons given below:
A. Fly ball wear
At idle speed, the opening degree of flying ball is the smallest, and the spring slider overreaches the flying ball because of the wear of the small roller of the ball, which results in an irregular direct collision with the fly ball body, resulting in instability of idle speed. At this point, touch the joystick with your hand, will have a slight feeling of being hit.
B. Elastic variation or improper adjustment of idle spring
When the diesel generator is in operation, the increase of load will reduce the speed. If the idle spring or start spring is softened, the feed rod can not move to the direction of increasing oil quickly to increase the speed, which will cause the diesel generator to stall automatically when serious.
C. Improper adjustment of steady speed spring
In idle operation, because of the small centrifugal force flying ball, speed control force is also small. If the diesel generator decelerates suddenly, the adjusting movement of the fuel supply rod may exceed the idle position, causing the diesel generator to stall. In order to prevent this kind of situation, the stable spring which is pushing the fuel supply tooth rod to idle position behind the governor cover; if the spring is too soft or the adjustment is biased, it will weaken or do not play a stabilizing role, making the idle speed running unstable.
More information at this article:
https://www.dieselgeneratortech.com/diesel-generators/Reasons-for-Unstable-Rotational-Speed-of-Diesel-Generator.html