Jim,

I don't own an MDS engine, but just from reading your post, I would say you are woefully under propped with a 12.25x3.75. The .68 is a fairly large displacement so that sounds like the equivalent of running a 9-4 on a .46.

As a general rule, the larger the displacement, the lower the peak rpm. That is, unless you are talking about the specialty engines like the Jetts.

I have run 14-4 props on .61s when I want a low pitch prop. Try one of these or a 13-6.