I had noticed, the past month or so before I sold it, when the engine was sold, and you started it up, the RPM gauge would bounce between 900-1350 RPMs quickly for several minutes, and the exhaust would be coming out in "spurts" almost as if whenever each cylinder fired, it would send a violent spurt of smoke out the tailpipe. It wasn't a continuous stream, but violet spurts, as if mimicking the firing order on one cylinder, perhaps. Also, it would make a "pop!" noise once in a while. Not like a backfire (but it probably was) but it sounded like it was in the catalyic converter. Sounded like a popcorn popper whenever it did it.
It doesn't matter now, but, any ideas on what that may have been? It's bugged me.