In The City of Ember, why is Don's first look at the generator disappointing to him?

Working as a pipework laborer has convinced Doon that the City of Ember is in much worse shape than he thought. This is due to the fact that the malfunctioning water system keeps breaking down. But Doon realizes that the pipework system is somehow the key to saving Ember from imminent destruction.
So he makes a beeline for the big Generator to see if he can figure out how it operates. He believes that he can discover how the Generator functions simply by looking at it. But he can't. For one thing, the Generator is absolutely gigantic, a two-story beast of a machine that constantly vibrates. His disappointing first encounter with the Generator puts a slight dent in Doon's confidence. He'd always been so proud of his mechanical talent, but in the face of the Generator, he can be forgiven for being a little less sure of himself.