@Denny I'm also thinking it may be too much pressure for the tube. I use a mix of tubes and tires throughout the year, and have not been diligent about recording what tubes work with what tires (and the recommended tire pressure). I just ordered a replacement set of Kenda K80 tires which run at 35psi max and will try them with the existing QTubes (26" x 2.1-2.3") instead of the current tires which recommend 55psi. I've probably overcomplicated something pretty basic, so we'll see what the results are once the tires arrive.