How to Modify Lawnmower Engine for Racing
Unless you are racing in an Open Class race, it is best to consult with the rulebook for your particular racing class before making any modifications to your lawnmower engine. Open Class will allow any modifications so take advantage of the sanctioned modifications and change out many engine parts to increase performance to the max.
Changing out the parts on a lawnmower engine will be like changing out the parts on a mini racing car.
1. Remove the governor. If you are unsure of the location, refer to your tractor’s repair manual. Overbore and sleeve the piston; this can increase your RPM into the 6,500 range. Choose a special order piston made of magnesium.
2. Purchase a racing stock carburetor custom prepared at a carburetor shop or look for a local racing go kart dealer. Remove the current carburetor and replace with the racing stock part. This will save you endless hours porting and polishing the current carburetor to increase airflow. Port and polish the intake runners of your engine head.
3. Boost engine performance with an increase to the lift and ramp speed. Replace the camshaft with a racing camshaft. This will allow the camshaft to open and close quicker. Two of the most common camshafts used in lawnmower racing are flatheads (cheaper and smaller and easier to fit onto a smaller chassis) and overheads
4. Replace the factory cast iron flywheel, which is prone to explode at high speeds with a computer-milled billet aluminum flywheel. The billet is lighter and decreases the overall weight for the motor to pull. The flywheel is nearly indestructible at high speeds. The United States Lawn Mower Racing Association is demanding it for all classes starting in 2011 for those very safety reasons. A billet aluminum “dogbone” connector rod should then replace the factory cast iron “dogbone” and thereby keep the rod from wiping out your crankcase at high speeds
5. Locate the muffler on the engine and shake it to make sure it is tight. If not, tighten it using proper wrenches as if it’s loose you are not getting proper backpressure to your motor and performance will suffer.
You should now use nothing lower than 92-octane gasoline in your modified engine.