From the makers web site, and of course a price premium as well:
Q: What is the advantage of a Premier Carb over a standard MK1 Concentric?
A: The 600 & 900 series MKI Concentric carburettors for four and two stroke applications has been re-designed to improve performance and wear.The new carburettor incorporates changes in materials which reduces the rate of slide and body wear and a precision engineered idle circuit. This improves the pick-up performance from idle and reduces fuel consumption. The precision manufactured forged alloy, Hard Anodised Throttle Slide has a low friction surface for smoother operation and greater wear resistance. The ethanol resistant, puncture proof StayUp® Float and Viton tipped alloy Needle Valve improve control of the fuel flow.
A new easily removable Pilot Jet provides precise metering of the idle circuit. The Jet, which is located in the opposite position to the Pilot Air Screw, is manufactured to tight tolerances allowing it to be replaced with minimum disruption to the adjustment of the carburetter. Initially the size of the jet replicates the 622/107 Pilot Bush. Removal of the Jet allows access to the rest of the idle circuit for servicing.
The Premier Pilot Jet cannot be fitted to earlier versions of the Mk1 Concentric©. This range is available in 22, 24, 26, 28, 30, & 32mm bore sizes and with right or left hand pilot/idle adjustment. The carburetters can be ordered by adding “Prem” to an existing specification number.
