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.
When the solenoid quits working, the engine continues to run because the computer allows the carburetor to run in "full rich" condition as it would without any emission control. This allows you to continue driving until you can have it repaired. With the engine getting so much extra fuel, you might experience a loss of power.
Have your mechanic check for fuel leaks, and if he can find none, have the solenoid checked.
Q: I recently had some carburetor parts installed in my 1969 Volvo when the car was tuned up. It runs fine on level ground, but it backfires badly when coasting downhill. What causes this? --J.H.S.
A: The backfire noise that you hear is the fuel igniting in the combustion chamber while either an exhaust valve or intake valve is still open. The fuel is supposed to burn when both valves are closed. If one is open, the air-fuel mixture explodes into the opening.
A common cause of backfiring when coasting downhill is that the throttle on the carburetor, which lets gasoline into the engine, is closed too tightly, blocking the flow of air through the engine. Your carburetor has what is called an idle compensator, which lets air into the engine when coasting to help reduce emissions. Have the carburetor checked.