I had an MDS 61 that ran fine for 2 years then started getting finicky at high speed. As I recall the consensus at the time was an air leak at the carburetor. Seems the machining on the early 45s and 61s wasn't quite precise enough to get a good seal. Or at least that's what was going around on the newsgroups at the time. Mine was still under warranty so it was replaced with a new MDS 68.