1988 Pontiac 6000 low MPG on '88 6000 L4 - Throttle Body

Hello,

I've replaced my computer, air filter, sending unit, fuel lines, and fuel filter. I only get 15MPG - I know I should be getting around 30. The vehicle is missing it's muffler. The tires are good, and I'm not a lead foot. I am trying to rebuild the Throttle Body now, but I really need to get this running good.