right, ok let me explain, in england it tends to be that the slower karts have clutches with of course a few exceptions, many of the high power karts don't have clutches, theses are called "direct-drive", these karts just have a sproket on the output shaft and one on the axle with a chain connecting them, therefore the compression from the engine is sent straight to the wheels. So to start the kart you push it until the wheels turn enough for the ignition coil (also connected to the crankshaft) has enough kinetic energy to create electricity for the spark plug to fire and then the rest is simple, you just throttle it so that the fuel starts flowing and off you go. To cut out the engine you simply stop the kart, because the wheels are connected straight to the engine, putting the brakes on hard so that wheels stop turning stops the engine turning over. this is why i think using the ignition cable to detect revs is the best solution.**broken link removed** THIS is a very similar ignition system as the one on my kart. My engine is a Parilla tt75 racing engine, its 100cc and pushed around 13-15bhp (i forget the number exactly)