It controls the amount of air entering the engine at tickover by moving the valve closer or further from the valve seat in the Plenum Chamber. The problem that occurs with the stepper that can cause idling problems is that it gets a build up of carbon deposit on the valve which then starts sticking. 

The stepper is first port of call and normally sorts the problem, but as with everthing in life "not allways" so a couple of other basic things can be done. 

If this doesnt cure an idling problem there is also a test you can do on the wiring shown in Steve Heaths The TVR Griffith and Chimeara book.

Also the Throttle Butterfly in the Air Intake to the Plenum can also get Carbon build up on the edges of the butterfly which can stop the butterfly closing properly, This can be cleaned by taking the air intake pipe off  and openngthe throttle and a small rag with white spirit on attached to a long screwdriver then delicatly wipe the butterfly. I cant stress delicatly enough as the Butterfly is very delicate and bending it wil cause more problems.

Stepper Location

Plenum with locations

To take the stepper out you must first disconnect the electrical connector which on mine was just push fit with a little clip. Once that is off you can unscrew the Stepper, Anticlockwise. Care must be taken if having use a large spanner or wrench as the plastic body can be brital and WILL BREAK very easy. Once undone and pulled out you will be able to see the valve and it will more than likely be covered in a Carbon Compound.

Dirty Stepper

Now its out it can be cleaned, I used a cloth and some white spirits/petrol on too clean the carbon off the valve. If the carbon is too stubborn then a wire brush or wire wool can be used to help get it off then wipe clean again with White spirits, or petrol. The appaerture that the stepper screws into will also require a wipe with a cloth making sure not to push any carbon into the Plenum chamber.

Clean Stepper

Once that has all been done you can now put the stepper back in, I use some PTF tape on the thread of the stepper to help  get an air tight fit.  BE VERY CAREFULL when doing back up as I said before The plastic casing wil brake easy, I just nip it up with a spanner but some people just do them up hand tight.

Below are a couple of images from the inside of the bracket that the Stepper screws into.I had to take this off when changing my Plenum. You can see some Carbon Build up allready around the valve and on the inside of the bracket.

Stepper In Situ

Stepper in Situ Closeup