why bad fuel use and poor acceleration

If the fuel lines are still hooked up to the rear tank you may be splitting the fuel from the engine and filling the rear tank at the same time. The fuel pump has a check ball that prevents fuel from going back into the unused tank, those check valves went bad enough for ford to have a recall back in the 90's for an inline one way valve to be installed at both pumps outlet on top of the tank.

fuel lines to back tank plugged off.fuel mileage came back(?).idle uneven.fast acceleration like driving through peanut butter (sometimes).if i accelerate easy it's better.once up to speed,if i let off the gas and then accelerate it will hesitate(sometimes).cat?-02 sensor? exhaust stinks different.

What is the condition of the engine? If the compression is low no amount of tuning will bring back the power.

FoR is right, if the engine is sour then no amount of work will help. You may be having a cat issue also. Try breaking the exhaust loose from the manifold a bit or removing the o2 sensor to see if it picks up. If it does then the exhaust is restricted.

don't know compression but replaced plugs and old ones are beautiful light brown.engine runs great at higher speed . will check o2.thanks guy!

pulled o2-black as coal.cleaned it with brake cleaner.shot some in o2 hole.my bad.wont idle but will start with pedal on floor.runs rough.cats?