Recently rebuilt my carb (Q-Jet) and now I have a backfire through the exhaust. I changed plugs, wires, dist. cap, and rotor and that helped a bit, but it's still there. Also, since I rebuilt the carb I have a hollow exhaust note and loss of power just before the truck shifts into second gear. Could it be that the mixture screws aren't adjusted properly? If so, how should I adjust these?