SU Carbs - Poor idle quality 

Something that crops up when any modifications are made to an engine - mainly in the form of improving induction and exhaust capability - is that of idle quality. After applying aforementioned freer-breathing products to enhance power output, many complain that no matter what - including post rolling-road tuning - they can not get a satisfactory, steady idle setting. And this after checking for manifold/gasket air leaks and spindle to carb body play (usual suspect as the spindle wears quite dramatically over several thousand miles). There is one thing that is often over-looked - the air valve found on many later model cars. This sprung-loaded air valve is built into the butterfly, sometimes called a 'poppet' valve or anti-dive valve. 
It is primarily there to reduce emissions when the engine is on the over-run - closed throttle at speed, i.e. anything other than at idle and gearbox in neutral. Basically caused when you take your foot off the accelerator when approaching junctions, downshifting gears and so on. 
When the throttle is closed suddenly with the engine at higher rpm a massive and sudden vacuum is created within the induction system down stream of the butterfly. This instantly draws any fuel stuck to the port and manifold walls into the cylinders causing a short burst of extreme mixture richness. The consequence is high exhaust emissions. The valve rectifies the problem by opening at these high vacuum periods by reducing the vacuum and bleeding extra air into the induction system creating a more oxygenated burn resulting in far lower emissions. The design flaw here is the spring - they deteriorate with age so no longer hold the valve tightly shut, thus leading to an inconsistent air leak causing erratic and rough idle. The other hassle is that this thing sticks out in the air stream quite considerably, significantly reducing airflow capability and therefore power output. It certainly explains why some folk don't get the 'as advertised' improvements a stage one kit is supposed to give. 
First you need to asses whether your car has one fitted or not. Not a telltale sign, but cars fitted with these valves (unfortunately I haven't been able to pin down when or where exactly these things were fitted as standard) exhibit markedly less engine braking on lift-off over the solid butterfly types. Seeing is believing - so a visual check is needed. Without actually removing the carb the best way is to remove the dashpot and piston, peer down the carb towards the butterfly and open the throttle by hand. If you can see a spring sticking out of one side of the butterfly you have that pesky valve. 
What to do about it? Remove the carb and either remove the valve and spring then solder up the holes in the butterfly, or replace the butterfly with a solid, non-valved type. Only takes two screws to sort it out. 
Useful part numbers: 
Note - all below are for 1.5" carb only. For other carb sizes, see inventory. 
WZX1323 Butterfly disc with screws (non-air valve type) 
WZX1325 Butterfly disc with air valve and screws 
WZX1177 Throttle spindle 
RTC5965 Butterfly screws (two needed per carb) 
AUD3080 Spindle repair bush (two needed)