Why doesn't Fortunato want the narrator to go to Luchesi?

Fortunato does not want Montresor, the narrator, to go to Luchesi to get an evaluation of the rare cask of Amontillado for a couple of reasons. First of all, it is reasonable to assume that Fortunato possesses quite a lot of arrogance regarding his own abilities as a connoisseur of wine. He clearly considers himself superior to Luchesi in this regard, as he asserts that "Luchesi knows nothing about wines, nothing at all." Therefore, one reason that Fortunato doesn't want the narrator to ask Luchesi about the Amontillado is that it feeds his ego to be the arbiter of the sherry's quality.
Another reason Fortunato doesn't want Montresor to ask Luchesi to taste the Amontillado is because, as the narrator observes, Fortunato "liked to drink good wine." It is reasonable to assume, then, that Fortunato really would like to drink a first-rate sherry and doesn't want to pass up this rare opportunity.

Fortunato says that he does not want Montresor, the narrator, to go to the other wine expert in town, Luchesi, because "Luchesi cannot tell Amontillado from Sherry." In other words, then, Fortunato says that Luchesi's expertise in wine is not as good as his own. This is why he says he does not want Montresor to go and request help from Luchesi.  
However, Montresor knows that Fortunato will not be able to resist gloating over Montresor's having been tricked into purchasing a great quantity of wine that cannot possibly be the rare amontillado. Even now, Fortunato says, "You have been imposed upon." Fortunato is sure that Montresor has been taken advantage of, and Montresor knows that Fortunato's pride in his connoisseurship in wine is his one "weak point." Fortunato's pride will compel him to follow Montresor into his niter-encrusted vaults, despite his bad cold and cough, so that he can revel in both his own discernment as well as Montresor's bad judgment.