Your D-Jet SL should give you 12-16 mpg, in-town vs highway. That screw is only for idle speed and you should set it to get about 750 rpm at idle. If your mileage is really really bad, check for vacuum leaks...there should be none anywhere. Replace all the vacuum hoses if they are hard or cracked so there are absolutely no vacuum leaks. The D-Jet systems depend on accurate vacuum to operate. Without good vacuum it will really suck the gas.