My original cat with the original engine was clogged, causing poor performance and overheating especially when climbing hills. After looking at everything else trying to cure this, I brought it to a friend's muffler shop in town and we sawed off the tailpipe. There was a thick black gooey mess clogging honeycomb passageways when we looked inside the cat. There was no excess noise in my case. We junked it and replaced it with an "Alabama cat" as shown below, and all was well afterwards. The cause? No idea. It wasn't using any oil, and it remains a mystery.