It's a damper on the piston. Fill it to the step inside with a mix of 1/2 auto trans, 1/2 CRC. Screw the top back on just loosely and reach inside and push the piston all the way up. Allow any excess oil to escape out the threads so the piston can reach the top, then just finger tighten the knob. Fuel mixture is adjusted by raising or lowering the seat that the big needle on the bottom of the piston slides into. There's a nut under the bottom of the carby that you can click in or out for best running.
Or just walk away, mini's will make you mad, as will anything else made in G.B.