Water or nitrous!
Since most of us don't have nitrous handy, except maybe Luke, use water. Start and warm up the engine. Trickle a small steady stream of water into the carburator while holding the throttle at fast idle. I use a spray bottle. Do it until it starts to bog, then stop and let it catch up. Do it a few more times, then change the oil. You will see a big black spot on the ground by your tailpipe and the combustion chamber will be quite clean. Just don't use your garden hose, you will hydrolock the engine if you use too much water.
