First a little theory.
The SU carb uses engine depression to raise a piston in the Dashpot. Attached to the piston is a shaped needle and the piston itself acts like a valve for the air. The higher the piston the more air is allowed in and as the needle is pointy the more fuel gets in. Its a big balancing act between the fuel and air and the shape of the needle determines the ratio. It is important to remember here that the amount of depression does not depend on rpm, but on the load of the engine, i.e how much air it is gulping.
In a turbo HIF this is slightly modified because now everything is under pressure from the turbo. The neat way they got around that was to install a small restrictor plate in the plenum, this ensures that the pressure before the restrictor is slightly higher than inside the carb throat. That slightly higher pressure is fed through drillings firstly into the float chamber to "pressurise" the fuel. and secondly to just below the piston to raise it.
Note** the fuel pump and regulator DO NOT give the fuel inside the carb any pressure, the regulator is just there to make the fuel pressure 4 psi or so above the pressure inside the float chamber so that fuel can DRIP in to replenish what is used, this way the little valve is not overwhelmed by 16psi of pressure.
The Dashpot spring determines how high the piston rides for a given depression. You should use the lightest spring that allows the piston to raise fully to the top at WOT to ensure quick response to the throttle. If it gets to the top before WOT or never gets there then you have the wrong spring. I am investigating other ways of seeing this in the other pages of the tech tips. The standard 8oz should be fine, maybe a 12oz just adjust the profile of the needle to suit it. Some people use double springs (16oz) I tried this, It was almost impossible to stop them binding and the engine was less responsive (a slug!), therefore with 8-12psi I would not even think of using a double spring.
Dashpot oil is used to enrichen the mixture in a snap open throttle situation, the oil resists the piston lift and the resulting lower than normal piston height "chokes" the engine and causes more fuel to be sucked in until the piston reaches its normal height, then the balance is restored. Start with a light oil then keep trying heavier oil till it snaps open without hesitation, Standard SU dashpot oil is 20wt. Another thing to keep in mind is that the oil does affect the WOT mixture, with heavier oils the piston is almost always lagging a bit and this makes the mixture richer until it catches up, and it probably never will whilst you are changing gears.
And finally on a turbo carb there is a part throttle lean pipe on the carb, I found that removing and plugging it enrichens the mixture quite a bit but with it installed the cruise mixture seemed more stable, In the end though I found that the mid throttle fuelling was better with the pipe disconnected and plugged.
Next you need to work on the next stations. I found it best to use a light oil in the dashpot at this point so that the readings were more accurate.
Without a sealed lift indicator you are going to have to guess. In my case light throttle was stations 6-7. then go for a 3/4 throttle acceleration station 9-10, and finally full throttle which should be around station 12 unless you have the wrong spring fitted.
With a sealed indicator its a case of holding the throttle and accellerating just enough to keep the indicator at a station and once it stable take a quick look at the O2 sensor and you will see if its lean or rich, polish accordingly. I found that 0.03mm was giving me about a change of 1 on the AFR. I also had trouble seeing the numbers on the lift indicator so I used a small tiewrap around it to show what station I was looking for and just slid it up and down as needed.
You will probably screw up the first needle! but every time you try this you learn what stations do what and the next one is always better, stick with it.
Polishing needles takes a big learning curve but here are some of the things I found with my setup.
I needed quite a step from station 5 to 6 to prevent leanout when pressing the throttle from cruise, the position of this step depends on a lot of factors but it may be that because I am using a small light turbo that spins up easily it was moved up the needle. The dashpot oil you use has a big effect on the WOT AFR. if its thicker, the piston never really gets to its final position in the short time you can hold the throttle open. Because of this you may see some leanoff in top gear when it finally catches up. When I had the leanoff pipe connected I also saw problems with a leanoff when I held the throttle open just enough to give full boost, in this case the piston does catch up and it highlights the lean needle you really have, any attempt to fix this caused overrich mixtures at WOT. In the end I went all the way down to a 10wt oil to get the balance between this and WOT to be reasonable but not perfect. This is why I would suggest blocking the leanoff pipe..