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Re: increase power/torque of a 2 stroke
By FAR the most important addition is a tuned exhaust, and, yes, your first post is spot0on for the basic theory.
Lawnmowers have virtually no tuned exhaust, compare its exhaust to a small dirt bike. You want to duplicate the tuning the expansion chamber gives you.
Possibly there may be kits out there, or you may be able to adapt a small 2 stroke dirt bike exhaust to the lawnmower engine, although porting would have to be modified for optimum results.
IMO you are far better off to use a dirt bike engine and trans, and build a 'shifter' cart. Since the manual trans and clutch are integral to the engine, its actually quite straightforward to build.
BTW a tuned exhaust can easily increase power by 30-50%. I know first hand by having the exhaust system accidentally fall off my 2 stroke dirt bikes and snowmobiles while riding. Noise increased a LOT and power decreased substantially.
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