I just finished up with this homemade gokart with a harbor freight 2.5hp engine. We have a small backyard and my boys have never driven one before so I was trying to have it go around lawn mower speed. I'd say it probably goes 10mph when topped out. The governer is removed and it will wind out with me on it to over 4500 rpm (I have a tiny tach). I used a 12 tooth clutch and then took a spare jackshaft sprocket and welded it to a 40 tooth axle gear which is on the jackshaft. Then it goes to a 13 tooth sprocket and then to a 60 tooth. Frame is welded out of 1" tubing from local hardware stores. It was very tricky because of the thin tubing and avoiding blowing through the metal with the welder. This engine is capable of much faster speed with different gearing however my family has a yard with some hills and the boys are cautious about putting the pedal to the metal so the low gearing works fine. It did take me several head scratches to figure out why the engine kept shutting down and then figured out it had a low oil cutoff that was triggered when I was cutting the corners. I actually built two karts in two weeks. First one was a 1.5 feet longer which was great for adults but even with the adjustable seat the boys were having a hard time pushing the gas all the way down.
The 60T sproket was swapped for a 40T sprocket on the main axle for additional ground clearance. The current drive is a 12T clutch to a 40T on the jackshaft outputing to a 11T and then to a 40T on the axle sprocket. This gives a final drive ratio of approx 7:1. I clocked it with my gps at 12-13mph downhill and 11-12mph uphill yesterday with my 215lbs on it. Climbed some fairly good hills with no problems. We hooked a radio flyer wagon to it and I pulled my 11 year old son and 4 year old daughter in it at the same time (an extra 110lbs) with no problems.
Since this is a popular video I figured I'd report back that the kart is still alive. Both kids and adults ride it throughout the summer. Even had it in the snow once or twice. Only real problems we have had is the allen screws occasionally backing out of the sprockets (now using red Loctite instead of blue). Engine still runs good. Have gone through a couple of sets of tires. Other than that its still a blast and the frame has stood up fine.