Find the throttle cable and where it connects to the fuel pump, below which is a 'screw' that can be adjusted up and down to increase ir decrease the idle speed. If this isn't enough, PM me and I will get you some photos.
There are potentially two adjustments, one in the cable and the other on the engine. Disconnect the cable at the engine end, adjust the screw on the engine if necessary to get the correct speed, then reconnect the cable - you my need to adjust the cable attachment so the cable is not in tension.
