Below is an extract from the SU Carburettor Manual downloadable at https://mgb.tips/2019/02/09/downloadable-tuning-su-carbs/
The procedure also applies to HIF carbs as well.
The only comment i would make relating to these instructions is not to use the button on the side of the carb as it’s just about impossible to know you’re lifting the 1/16″!!. Better to take air filters off and lift the piston with your finger or screwdriver so you can see piston rise …….. it’s easier to judge the 1/16″.
The next best way i find to check fuel mixture is by looking at the colour of the spark plugs after a decent run in the car …….. see https://mgb.tips/?s=spark+plugs
Note: To richen mixture on HS4 carbs screw the mixture screw out, on HIF4 carbs screw in (clockwise)
MGB tips mods and maintenance 