Walbro on a stock KA is unnecessary, and if your stock FPR isn't operating right, you have WAY too much fuel goin in. check fuel pressure. Also you could have some leaky injectors, one leaky injector can kill your fuel mileage. EGR helps with gas mileage at cruising, so that will hurt you a little bit...but you have a MAJOR problem with the AFR. Check the 2 things i said above. Exhaust shouldn't cause a problem, as long as the engine has SOME back pressure, and the O2's hooked up.