I have a John Deer RX75 that would only move at the slower speeds and to make it behave you had to put your foot under the pedal and pull up. I tried to adjust it under the seat and now it won't move at all! Can someone help? Apparently I know just enough about it to screw it up!

As I recall that mower is not fun to work on. The linkage from the petal to the spring loaded pullet is gummed up. Get under and watch as a friend moves the pedal, spay WD 40 or Liquid Wrench on all pivoting cotact surfaces, keep spray off the belts.