I second the "block of wood" suggestion by Forced.
On the older models, pre-electronic actuators, you just made a simple adjustment to the linkage, which restricts how much fuel can be delivered, thereby decreasing peak hp. Supplement that with a block of wood. There ya' go. Max mileage.
Who cares if your top speed becomes 65 - 70 mph and it takes you a minute to get there. Isn't that the limit anyway, and what's the rush?