The problem with attempting to adjust the air/fuel mixture at home is that there is no accurate method of objectively testing the results of your efforts unless you possess one of the small exhaust gas analysers which are today available for the DIY motorist. Now that exhaust gas is measured as part of the UK annual MOT test, it may be as well to ask that the mixture is set by the tester before your car is tested. Otherwise, it should not take too long nor cost too much to have this small job carried out professionally every six months. Even accurately setting the tickover (750 rpm for most models and 850 rpm for cars with semi-automatic gearboxes) requiresthe use of specialist equipment. However, if you wish to do the work at home, these are the basic principles. Begin by warming the engine then adjust the throttle adjusting screw (which bears against the throttle lever) until the revolutions rise to just under 1000 rpm (fast idle). Then turn the mixture control screw slowly clockwise until the point at which the engine begins to run erratically (the mixture is weak) and turn back by 60 degrees (one third of a complete turn). Re-set the tickover. As stated, it is best to have this work carried out by a professional with the aid of an exhaust gas analyser, and you would normally only carry out this adjustment following an engine or carburettor rebuild, in order to get the engine running well enough to make it to your local garage!In addition to the home exhaust gas analyser machines now available, a number of devices are advertised to make setting mixture easier. Chief amongst these is the Colourtune system, which uses a special spark plug which allows you to see the combustion colour and adjust the mixture to obtain the correct colour and hence the appropriate mixture.Still on the subject of fuel, check the fuel tank and lines for leakage.