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I'm no expert on SUs (or any carbs, for that matter) but a mate does a lot with MGs where they are widely used, and I recall some talk of a specific type of needle valve which was all the rage but turned out to be quite unreliable. At least with SU spares are plentiful.

Or the ethanol thing is possibly a point, too.

Yes you're correct Mike the SU can be fitted with an aftermarket needle valve with a teflon tip. They work very well but fail completely when worn a tiny bit.

Sounds like the SU carb needs a new float needle valve. This is a common problem on SU carbs and is very easy to fit.