Inspect the gasket surface

Inspect the gasket surface of the float chamber carefully for damage. The gasket surface should also be inspected for flatness. Light damage and irregularities can be remedied by dressing the surface using fine abrasive paper on a flat surface, or fine grinding paste on a plate of glass. If the surface is reduced too far however, the depth of the slot for the float spindle will become too shallow and may cause difficulties with the operation of the float.  Over-tightened Allen bolts, which are a popular modification, can bend the lugs and cause an air leak within the float chamber into the idle circuit. If the lugs are bent the float chamber should be replaced. 

Ensure the two galleries in the casting are thoroughly clean.

Clean and inspect the float chamber drain plug which acts as a sump for the float chamber. Replace it if it shows signs of hardening with age. Early versions of the Mark 1 carburetter were not fitted with drain plugs. These carburetters were fitted with a brass mesh sleeve filter fitted around the Main Jet and replacement with the later type of float chamber is recommended.

The float needle bears against the seat at the bottom of the brass seating bush. If the  seat is damaged the float chamber will have to be replaced. Early models of the Concentric were fitted with plastic float needles which should be replaced as a matter of course with the current Viton tipped float needles. Viton tipped needle valves are much more resistant to wear but should be replaced if there are visible signs of wear, contamination, or damage.

If a punctured float is suspected, submerge the float completely in fuel for 15 minutes and shake it to see if any fuel has leaked into it. Check the float tabs for signs of distortion, and check that the float spindle is an easy rotating fit in the float with no stiffness or excessive play. 

The All New AMAL Concentric StayUp Float

The AMAL Carburetter Company, (Burlen Fuel Systems Ltd) manufacturers of all genuine new AMAL Carburetters and spares Worldwide, have introduced a revolutionary new float for Mk1, Mk1.5 and Mk2 Concentric AMAL carburetters.

The new adjustable height StayUp float is resistant to modern ethanol based fuels and has a military spec closed cell construction making it puncture proof. The ability to adjust the float arm allows for quick and easy minor alterations to float levels where required.

The material specified was used by the S.U Carburetter Company for military installations in the 1980s, where a guaranteed fail safe operation was critical. Due to the close link between S.U and AMAL today (both owned by Burlen Fuel Systems Ltd), the secret behind the material has been released and upgraded for the manufacture of this float, making it virtually unsinkable.

INFO FROM HERE
http://www.amalcarb.co.uk/