Right, First do you know where your auto choke is in the car? on models after 1988.
If not look at the front of the car (engine bay open, lol) you can see the injector manifold. through that you will see the accelerator cable poking through. (from here you can rev the car!) now once you found that section, look back towards the front of the car (towards the rad) you will now see a flat head screw.
Get a flat head screw driver and adj till revs die down, be-careful as you bring it so low that the car won't idle at all.
Or
Replace accelerator cable, as it must have lost its strength.
Hope that helps
mishal
Mishal Chotai, May 2008