Combustion is never 100% used to drive the piston as a small amount gets by piston rings, commonly known as blowby gases. With the engine running, removing the oil dipstick may result in pressure relief as combustion blowby gases pressurizes the crankcase of every engine and blows out the oil dipstick hole. This is considered normal as long as the engine runs with few issues. The black exhaust may be the excessive fuel still leaking from the carb. I'm presuming this carb is gravity fed and the source of the leak is from the carb being fed fuel not stopping at the float needle. In general, the float is adjusted so fuel level is at or just below the venturi feed into the intake manifold. Vacuum from the engine rotating always draws fuel from the carb nozzle or pinhole to atomize into droplets, mixed with air for the proper air/fuel mixtures. If the float needle is damaged, bent, or misaligned, fuel may simply leak out as a dribble with the on level ground for observation. While not rocket science, carb float/needle adjustment is to ensure fuel in the bowl is not flowing out of the carb, into the intake manifold with engine off. Fuel should flow only when the engine's started/running. There are many sources of information about basic carburetor operation and adjustment. The lean/rich idle mixture control(s) are additional fine tuning of mixtures and requires specific information from owner's manuals or factory info. This shouldn't be secret as its needed for anyone maintaining their machines without paying a dealer or repair shop for information. Some repair shops may provide information freely as a way to encourage returns for future parts when needed.