The driver heard a yell for a taxi, and he reaches up to activate the brake to slow down
below are the many different controls the driver has, you can see he pulls the choke
Meanwhile in the engine bay, Stimey (the black kid before Buckwheat) is making engine noise by rolling a cam against the "ingine" stick
Here is the short piece from the 20 minute short movie "Free Wheeling"
And here is the full movie, and the taxis scene starts at the 5: 32 minute mark, and then skip to 8:48 for the refueling of the donkey, and 12:01 for the drunk donkey driving