A four-stroke engine fires every other crankshaft revolution. Each cylinder creates its own pulses. The exhaust manifold tries, with varying success, to blend

A four-stroke engine fires every other crankshaft revolution. Each cylinder creates its own pulses. The exhaust manifold tries, with varying success, to blend those into a single higher-frequency pulse train. The Liquid rotary fires thrice per revolution, or six times faster. The go-kart engine is designed for 10,000 RPM. That puts its fundamental frequency at a less objectionable 500 Hz. But the go-kart video did not sound like 500 Hz -- it sounded like about 100 Hz. As for the "alarming amount of fuel out the tailpipe" line, maybe they're comparing a two-stroke engine. I don't really know how unburned fuel affects a two-stroke's sound.