R H fuel tank, when I select from main tank to aux tank I get a 50 to 75 rpm drop in rpm and it feels a little rough.We checked the selector valve...OK We changed the fuel pump.We put fuel calibration gauges on it and tried to adjust it to the main tank. But we still get a drop in rpm when the aux tank is selected. We checked the R/H fuel selector valve for obstruction and appeared to be ok. Any sugestions???????????????????