SU carbs have a Yellow metal needle in a main jet that is raised by a piston to adjust fuel in relation to airflow. Much like the needles and sliders on M/C carbs.
When the engine is at rest, the needle sits mainly submerged in fuel.
Mohawk Gas in Canada for many years, advertised that their gas had added Methanol and Ethanol as 10% of the gas. My Buddy got a discount at Mohawk and over an 8 year period ran his MGB (with SU carbs) on this gas exclusively.
Over time he found that the car began to run quite rich, especially at idle (Black smoke) When the carbs were dissembled, a 'pronounced' ridge and pitting was found on the needles of both carbs, starting at the area where they meet the fuel.
I have never seen this before! I HAVE seen needles worn over many years from use (some needles were Biased, spring loaded) But the Pitting I can only put down to Fuel erosion or corrosion.