I built a predator engine from harbor freight, completely modified. Billet rod, flywheel racing cam, rockers, air filter and milled heads. This motor started out at 6.5 hp as it sits now..it easily does 50mph. This is what got me tinkering with small engines and led me to build a mower....that and the constant harassment from my father in law asking me when I was going to build one. I built the kart with the intention to race but I found the requisites to be a little extreme. Plus I would of had to build a new motor because I apparently started with the "wrong engine". Oh well handles like a dream and has great pick up.