Would eating mainly soluble fiber and no insoluble cause constipation?

It can: certainly CONTRIBUTE to constipation however the large bowel is perfectly capable of REABSORBING the WATER from the soluble (dissolved) FIBER THUS.....stool! Hope this is helpful Dr Z

No: Eating fiber does not cause constipation. In fact, consuming soluble fiber actually treats constipation as well as diarrhea. Some people take drug agents containing fiber to help with bowel movements. To prevent constipation, you should eat foods high in fiber such as oatmeal, apples, bananas, and vegetables.