The mixture "screw" is the BIG NUT with the BIG spring above it that is UNDERNEATH the carburettor.
The Jet - the thing with a red (probably, but possibly Black) plastic bit on the bottom of it, and with a tube going to the petrol float chamber - goes
through the centre of the BIG NUT that adjusts the mixture.
To learn how to tune the carbs, refer to this article
http://www.mintylamb.co.uk/?page=sutune.htm
The two extra tubes that go from the top of the float chambers are overflow tubes. On early cars, they were simply holes in the top of the float
chamber (well hidden, so most people don't know they were there). On later cars, there was an outlet to connect a hose, and on the latest cars, the
hose was fitted.
The purpose of the hose is to route any overflowing petrol away from the hot exhaust - it took a long time for Triumph to learn that petrol overflowing
onto the exhaust is a problem.
The "other end" of both pipes must be left open, and free of all obstructions, otherwise it will cause problems with flooding and overflowing carbs -
connecting them together just makes it worse !!!!!!!!