These jets will fit all Arreche/Amal carburetores up to 21mm.

The main jet controls fuel flow from 3/4 thru full throttle. Once the throttle is opened far enough the size of the hole in the main jet begins to regulate fuel flow.

Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). Higher numbers on main jets will have a richer air/fuel mixture than smaller holes.

To adjust the main jet it is good practice to start with an over sized jet. This will cause the engine to run very rich, but will ensure you don't cause any damage while adjusting. From there gradually decrease the size (in increments of 5) of the main jet until your engine runs properly. Once you find the size that works well you can make fine tune adjustments by trying different size jet around that size but using smaller increments.

Caution: Using too small of a main jet will cause your air/fuel mixture to be too lean and will result in a piston seizure.