The most common carburettors used on 100 engines are the Tillotson HL334 which has a 19.8mm bore and the Tillotson HL360 which has a 24mm bore.
Both of these carbs have 2 jets and a rough guide for setting them is as follows:
The low speed jet has a slot in the adjustment screw so it needs a screwdriver to alter it. This should be turned fully home (clockwise) and then unscrewed to one full turn; it may be possible to reduce this to about 7/8 of a turn in fine tuning once you get used to the way it works.
The high speed jet has a bar through it so it can be adjusted by hand; again you must start with it fully closed then unscrew it to between 1¼ and 1½ turns.
If the level in the fuel pipe drops down when the engine is not running it is likely that the carburettor needs overhauling.