My first new car was a white 1500 Spitfire in 1975. I was pretty excited owning a new car and the amount of time I spent lavishing attention on it was probably verging on the obsessive. My enjoyment was pretty short lived as it soon developed exactly the symptoms described, especially coming to a stop the engine would race to 5/6k and had to be shut off. Many trips to the dealer failed to fix the issue and I had to take matters into my own hands and removed and stripped the carbs.
I checked the pistons, as I thought they could be sticking and replaced the springs with stronger ones. Checked the bores and meticulously cleaned everything.
Nothing helped, I even changed the needles in case one was bent.
By this time the car was about six months old and had started showing signs of rust around the front of the bonnet and the seam on the boot. My love for this car was diminishing rapidly and when I contacted the dealer was told ‘it’s a cheap car, what do you expect’.
I resolved to replace the carburettors to fix the problem, and removed the from the car. I then noticed something I hadn’t seen on previous cars. The butterflys had a spring loaded valve in them which was presumably to stop them shutting off rapidly. I found replacements without the valves, installed them and the problem was fixed (but not the rust!) This was British Leyland at its finest, I sold the car shortly after and vowed never to buy another BL car again, which I stuck to until a few years later when I bought a MGB GT V8, another love story with a sad ending.

Steve..bring back memories of my Brother in Law letting me borrow his Spit for a month..and it had more gremlins than my girlfriends..but when it ran it was fun as heck..Rich..

What a nice car, Steve. Brings me back to my younger years.

I have seen SU on cars but i didn't know how they worked  ! Thanks for the lesson  .
Kit from down under

Hi Steve, nice little car where it's easy to work on. Reminds me of my old Honda. Greatly filmed.😊🎥👍🏻

Double thumbs up because of the excellent explanation about the order of diagnosis and why that carburator would work or not.

Cool 😎

Sticking fuel float valves and or mechanical fuel pump issues.  These are common problems with these Triumph engines if they have been standing.