Don't chew with you mouthfull
In general usage the sentence would be, "Don't talk with your mouth full," (the understood "of food" usually omitted) and the prepositional phrase "with your mouth full" would function as an adverb modifying the verb "talk."
Mouthful is a noun meaning: "as much as a mouth will hold" and is incorrectly used in Rykat's sentence, as it is not possible to "chew" with a "mouthful;" one "chews" with their teeth. Even had Rykat used the verb "talk," the sentence would not have made much sense.
As a sidenote, Rykat also omitted the "r" off of what I believe he intended to be "your" instead of "you." Official ruling: short story disqualified.
AmbergrisCaye.com 
