I love SUs. So easy to work on, to adjust etc. etc. Mind you, you need to know a little about how they work. With multiple set-ups, a "floating" vacuum gauge is a "must have" tool in order to make sure that each carby pulls its equal weight. Adjusting the mixture is reasonably straight forward, and is usually achieved by lifting the "checking" pin. However, it can take a bit of time to get that right - the most accurate assessment is to run the engine, or drive the car, for at least ten minutes on a steady throttle, stop the car and remove four plugs (on a V8) and compare the colours of the plugs. A nice grey colour is what you need to look for. Regards Styria
I always found that a "Colour Tune" device was the most accurate way to confirm a Carburetor. The whole range could be checked ( read ) Idle.. acceleration.. cruise and full throttle. With the twin Carbs on my MG . Jaguar and a Riley, two Colour Tunes were necessary, in no 1 and no. 4 cylinders.
Even a very slight air leak down stream of the Carb made accurate adjustments
impossible..... Ask me how I know !!
Many years ago, John S. was good enough to pass onto me the aforesaid Colour Tune instrument referred to by Sean. I have to admit that I could never really come to grips with it properly - don't ask me why, but possibly I did not have enough patience. Admittedly, I tried to use it on the 3.5 litre V8 engine where access to the plug holes was fairly restrictive.
Instead, I relied on the floating vacuum gauge to ensure that each carburettor was drawing the same amount of air. Again, many years ago I purchased the SU Carburettor Tune book which is an excellent reference and instruction book.
N.B. I am very sorry, but presently I am unable to download pictures of the book and the Midel option of needles available. Quite frustrating. Regards Styria
Tuning an Engine with S.Us is as the song says "Start at the very beginning a very good place to start." First the Fuel pressure ,,Goldilocks....not to high or too low. The float level ..... the jet just lapping fuel level with the Carb. Body........Setting the level with a 7/16 rod under the Float lever can be a bit hit or miss. Also if the Jet is to low it will negate the book settings for the initial set up. Say two turns down or whatever. Even the slightest Air Leak after the carb will make it impossible to set the Carb........... Ask me how I know !!
Use a Colour Tune ( or Two ) as this will show just where the mixture is up the rev range..... maybe the wrong needle could be fitted.