Engine runs wide open unless brake pedal is pulled completely back. Why?

my 13.5 hp. yard machine riding mower runs wide open even if you move the throttle control to it's lowest setting. It will only idle down if you pull back on the brake pedal with your foot. I have had this problem ever since a friend of mine borrowed it. He said that he did something underneath because it was running to slow. I can,t get a hold of him to find out what he did. What could be the problem and how can I fix it? What could he heve done underneath the mower that would cause the machine to run wide open? I don't want to burn the engine up.