Tim,
Unscrew the black plastic cap on the top of the carb. Be careful, there is a spring underneath it, not a strong one but it's there. Try not to remove the spring you only need be concerned with the black cap. Turn the cap over and you will see a plactic peg about an inch long in the centre, that fits in the centre of the spring. Cut it down to about 2-3mm, just enough to still hold the spring in place, then replace the cap and fasten back on. That's it!
Phil
June 2009