I use the MasMo motor myself and found it to be by far the best engine I have tried. If your motor tune is going from severely rich to tuned or lean when warm you need to look for an air leak. It sounds as though once everything warms up and expands an air leak is developing. I had a similar problem with my Orion motor, I would suggest you check your backplate and, and all collaborator seals. Put the engine back to factory settings to get you in the ballpark and go from there.

Mark