Question: My 1983 Ford F150 pickup got 18 miles to the gallon of gas when it was new, but recently it dropped to between 9 and 10 miles per gallon. I've had the car worked on half a dozen times, but the garage people can't find anything wrong. What would cause the drop in gas mileage? --C.B.
Answer: Your fuel economy probably dropped off for one of two reasons: either you have a gas leak somewhere between the fuel tank and the engine, or your carburetor is feeding far more gasoline to the engine than it needs to operate properly.
Because your mileage dropped by half, you are losing about half of the fuel that you put into the tank. That would require a pretty big fuel leak, and you would most likely be able to smell gasoline when you park the car and turn off the engine.
If you don't have a leak, you could have a bad feedback solenoid on the carburetor. Most cars have a similar device that is designed to regulate the air-fuel mixture to the engine to achieve the best possible performance and fuel economy with the lowest possible emissions. Before cars had such devices, the carburetor gave the engine far more gasoline than it needed, resulting in a waste of fuel and more unburned hydrocarbons going out into the atmosphere.