Mixture screw is on the left side of the carburettor, just in front of the accelerator cable bit. It may have the tamperproof plug still in it, so you will not actually see the screw until that is removed.
Turn the screw anticlockwise to richen the mixture, however do it gradually (about a quarter to half turn) until you feel its right. You should prob get it put on an emmisions tester and then adjust it, then you know you will not be breaking the law and coking up your engine by running it too rich.
BUT i think its better to have it running richer than it is to be running to weak.
Will