I know in one episode with the saturday rewinder (or something to that effect) where Johnny and Sissy and Duckey and Missy all fall in love, Duckey and Missy fart. That is the only time I can recall.
In 2 episodes as far as I know:
"The Dog Days of Johnny" he burps at 1:22 and farts at 5:52
Then "Saturday Night's alright for Johnny" he farts 2 times, and Missy farts one time. Dukey at 1:06 and 6:00 then Missy farts at 8:52.