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Step 1
Dismantle the engine completely to access the cylinder, piston and piston rings. You will need to use professional equipment to increase the engine size; if you try to do this project with inadequate tools, you will end up ruining your engine's cylinder.
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Step 2
Bore out the cylinder, where the piston goes. The cylinder has to be perfectly smooth, so you have to have a machine shop (or access to one) to do the job right.
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Step 3
Install larger rings onto the piston to fit the enlarged cylinder, which will result in more power coming from your engine when you hit the gas.
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Step 1
Dismantle the engine and remove the camshaft.
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Step 2
Modify the lobes of the camshaft by increasing the surface area of the tapered end, which is the end of the lobe that pushes the valve open. Alternately, you may be able to replace with slightly larger lobes, but that's something of a hit-or-miss process.
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Step 3
Replace the camshaft and reassemble the engine. The increase in the tapered end of the lobe will increase your valve duration, which is the amount of time the valves remain open to allow fuel in or exhaust out. When your valves remain open longer, more fuel gets to the engine, which results in more power and speed.
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Step 1
Remove the carburetor from the lawnmower engine to access the main jet.
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Step 2
Modify the main jet of the carburetor to make it larger. Doing so will increase the fuel flow to the engine, which will, in turn, increase horsepower. Many engine builders use the same block for multiple horsepowers, so if you get more fuel to the engine you essentially get the larger engine for a much lower cost.
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Step 3
Be careful of heavier emissions outputs due to the increased fuel flow, and keep in mind that there are limitations to how much fuel an engine can take in.
