I mean't to say that if you had a carb which did not have a rich midrange problem before the pump, then you should not need a Perry carb! Yes you back off the hex head screw on the back of the pump. One way to do this is to adjust the carb as you normally would without the pump. Attach the pump and adjust the hex head screw until you get a mixture which is noticably richer than it was before. Then lean out the high speed needle as you normally would, and adjust the low speed needle for a good transition. The midrange should be as it was or just slightly rich. If to rich then back off the hex head screw about 1/8 of a turn and check the high speed, being sure that you can adjust it so that your engine is too rich, but then of course you adjust it to a proper setting.