Things You'll Need
After a few long summers of mowing, your riding lawn tractor can become very worn out and possibly even fail to run. When your mower engine breaks and isn't fixable, you will have to decide whether to buy a new riding lawn mower or to replace the engine. You have to remember that while a new engine will be nice, the frame, belts and components of the rider are all still old and they may not outlast the new engine. Replacing the engine does cost about half the price of a new rider though.
Locate the gas hose connected to the base of the gas tank and the carburetor on the engine. Squeeze the clamp holding the hose onto the carburetor. Pull the hose off of the carburetor and direct the hose into the bucket to catch the gas.
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Unscrew the screw that holds the throttle linkage onto the side of the riding mower engine, behind the carburetor. Use the proper screwdriver to unscrew it and then pull off the throttle linkage.
Unscrew the nut holding the red battery cable to the starter on the opposite side of the engine with a wrench. Pull-start riders will not have this red cable, as they aren't electric-start.
Unscrew the bolts holding the base of the mower engine to the riding lawn mower frame. Hold a wrench on the top of the bolts and a wrench on the nuts under the riding mower frame and unscrew them all.
Lean the engine forward. Pull the drive belt off of the pulley on the crankshaft under the mower engine. Pick the mower engine up and set it off of the riding mower frame.
Place a pipe wrench on the crankshaft area above the pulley bolted to the base of the mower engine crankshaft. Unscrew the bolt holding the pulley to the crankshaft.
Screw the pulley to the base of the new engine's crankshaft.
Lower the new engine onto the riding lawn mower frame. Lean the engine forward and slip the drive belt over the pulley on the crankshaft.
Screw the bolts back in around the base of the engine and through the riding lawn mower frame.
Reattach the throttle linkage, starter battery cable and the gas hose to the new engine.
Fill the gas tank with gas. Add oil to the full line on the oil dipstick.