How fast will an 8 HP engine go on a go cart?

The engine will run to the limits of it rpm (revolutions per minute).

The go-cart will travel as fast as the gearing and torque of the motor allows. Theoretically, you could get a nitrous kit for an 8hp lawn mower engine and do a one-time burst to like 50 HP and maybe 100 mph, before the engine exploded and your go-cart disintegrated. One of my sons manged to get his go-cat up to 66 miles per hour using a 12hp Tecumseh motor from an old lawn tractor. Scary, very scary, but very fast. The neighbors called the police.