Me and my dad have built a couple of go-karts together. The first one we built we used a chainsaw motor. Well, we messed with the governor (or took it completely off, not sure) and the thing ended up going around, I'd figure, 80mph. It was LOUD too. Instead of going put-put-put it went BAM-BAM-BAM!
Thing is I completely broke the breaks very quickly, and the thing would just keep going and going. Luckily, I had a kill switch attached to the wheel, and was able to glide it out safely.
After that, we just stuck with Briggs and Straton unmodified.