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I would like to up my fuel mileage on my 1987 f250 with 460ci and auto. I presently get about 8-10 but I notice the exhaust is black and sooty. The vacuum is 19 compensated for elevation. I pulled the vacuum line off the main port at the bottom front of the spacer under the carb to hook the vacuum guage to and after while it was still running I tried leaving for a few moments the large vacuum port open to atmosphere. I noticed no change in the way it ran so I put the plug back in the line the way it has been. This engine has been desmogged and runs pretty good other than the mileage. I have adjusted the idle circuit on the primarys as lean as I can and still get it to idle smooth and not stutter taking off from a stop. The ignition at idle is 10 degrees. The high speed is fixed jets and I don't know what they are , I assume they are factory it is the holley 4180 I believe that is correct. My thinking is if I can calibrate the amount of fresh air coming in under the throttle plate it will lean the high speed and low speed circuits by a varying amount depending on how much I let in under the throttle plate. Anyone have an idea on this? Thanks in advance.
 

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The black & sooty exhaust and the low MPG says that the engine is running rich for some reason. Be sure the choke is working properly. You may have to assure that carburetor's floats and needle/seat are performing as they should. Be sure the air filter is clean. After everything gets correct, the MPG should improve.
 
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