I had problems fitting a Ram mount for a Zumo 660 to KTM bars because they are tapered. The centre of the bars where I wanted to mount it were too large a diameter for the 27mm internal diameter of the supplied U bolt. I found that Radio Spares sell a U bolt with a 34mm internal diameter which fits over the bars but still fits through the slotted holes in the Ram part.
The U bolts came in a pack of ten so if the bar diameter is your problem I have 9 left and you are welcome to have one if you want one.
Looking at pictures of a k1100rs I can see it is more to do with the trim on the bars. However if anybody needs a U bolt they can have one of mine.