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.