There is a muffler place on 27th de Febrero, on the eastbound side, just past the Nacional supermercado at the corner of Ave Abraham Lincoln.
From Gazcue, go west on Avenida Mexico to the very end (which is a T intersection with Ave Abraham Lincoln). Turn right, and then at 27th de Febrero (the very next corner) turn right again. Just past the very large building on the right, (Supermercado Nacional and Ferreteria Cuesta) - look for a giant metal "Muffler Man" and a chainlink fence, and turn in there.
