Begin by changing pulleys
Change pulleys. Replace the original ones with smaller ones. This will give your mower more speed. Take motor off of lawn mower. Turn the mower upside down, carefully. Unhook dry belt from the pulley. Using an wrench remove bolt and tap that holds the pulley in place. Replace with smaller pulley.Inspect the dry belt. Look for signs of rotting or wear and tears. Replace if needed.Hook new or original dry belt back onto pulley.Make sure all bolts are tight.
Inspect muffler.Look for holes and corrison.Replace if you seen even the tiniest pin pole (it will cause you to loose speed).Clean away corrison.
Unhook governors.These can be found on carburetor. Some lawn mowers may have more than one governor. If they do, then onene governor leads to the gas feed and one leads to the throttle feeds.Unhook the one from the throttle feed.This will give you more power.
Inspect tires.Make sure you have proper air pressure.Some may choose to replace back tires with larger ones, with ruts, to make riding easier in mud and dirt.You may decide to keep the original tires in the front or even change with smaller ones.
Check guidelines of racing track you intend on trying.Some tracks requires that you leave the mowing deck on your lawn mower. The reason is that the mowing deck helps to keep the mower steady and level. This is especially helpful when you are racing around bends and curves.Other tracks may allow you to remove it.
Paint and decorate the riding mower as you wish. You may decide to even add a racking number onto it.
Check out other racing lawn mowers for more ideas on how you make your lawn mower race faster.