Ferris Evolution with an Engine Speed Issue

I have a Ferris Evolution with a 48" deck powered by a Kawasaki 27 hp engine. I have owned this machine for three years and have overcome shredding deck belt issues during the first year and now have a problem that has me scratching my head. They tell you to run the engine at full throttle when mowing and that used to work great until recently. The engine starts, powers up and mows great but as I mow, the throttle speed continues to decline, literally. If I move the throttle lever back...

Ferris Evolution with an Engine Speed Issue