No worries mate.
When the jet is loosened, it prevents vacuum from the intake pulling fuel through the jet, as the seal around the jet is broken, and air is simply pulled through instead.
Rev's can be adjusted in 3 ways:
1) Some models have an adjustable poppet inside the carburetor with the letters a, b and c on it, for most mowers it should be set to c.
2) Over time, the inside runs of the carburettor can wear out a little, this will cause increased rev's, to rectify this, a very small brass washer can be fitted to the central post inside the carburettor, below the throttle rotor.
3) On the throttle control/air filter you will see a small Philip's head screw at the point where the hose connects, this can be loosened, and moved up and down for minor adjustment, it should be set so that when the lever is pulled to the stop position, there is a matchstick width between the lever, and the end of it's running slot.
Rev's can also be affected by air leaks, at the bayonet carby mount o-ring, at the intake port o-ring, and the o-ring where the starter meets the engine block, also, muffler blockages, damaged head gaskets and problems with the decompresser operation can effect performance too.
The carburettor is essentially a pre-set unit, it is governed by a vacuum feed from the flywheel when in the "run" position, and pre set to "choke" in the start position, when in the stop position, 2 contacts are connected, shorting the ignition system.
You can find more detail about the construction and operation principals of the carburettor in the manuals section.
