1979 Ford Bronco Repair Question
Mile$ per gallon
I had good luck increasing the fuel mileage many years ago on my '72 Dodge Dart. First you must understand one problem inherent with all carburetors. Imagine a graph that shows gallons of fuel used vs. vehicle speed. With fuel injected systems that graph is a perfectly straight line which simply means the exact perfect mixture always enters the engine. The computer makes fuel metering adjustments based on road speed, load on the engine, coolant temperature, incoming air temperature, how fast is the throttle moving, which way is the throttle moving, etc. The mixture is infinitely variable but always near perfect with nothing wasted.
With a carburetor, there are only two places the mixture is perfect, that is at idle and at high speed. That straight graph line is still what is required for the engine to run well. If the actual fuel delivery drops below that graph line at any point, you will have a stumble or sputter. To avoid that, the jets are sized to always deliver enough fuel and the trade-off is there will be many times when there is too much fuel. That causes excessive emissions and loss of fuel mileage but won't be noticeable as far as performance goes. To state it a different way, there must always be enough fuel and to insure that, there will be too much fuel most of the time. The mixture can only be tailored perfectly at idle and high speed.
Carburetors also have accelerator pumps. That's what makes the fuel streams squirt into the air horn when you work the throttle by hand. When you press the gas pedal air flow increases instantly. Fuel is heavier and must build up momentum before it starts to flow fast enough. That lag causes a severe stumble upon acceleration. The accelerator pump pushes extra fuel in to overcome that lag. At highway speeds and gradual throttle movement, that pump isn't needed but it squirts extra unneeded fuel in anyway. That job is done electronically in fuel injected systems and the computer will not spray extra fuel when it isn't needed.
The first thing you can try is to adjust the accelerator pump. Most carburetors have multiple settings. You move the linkage to a different hole in the lever to make it take smaller strokes.
What I did on my Dart was to modify the spring on the power piston inside the carburetor. Vacuum holds the piston down against spring pressure. Two metering rods are connected that piston. When you accelerate, the lower manifold vacuum lets that piston to move up from that spring pressure and that pulls the tapered metering rods out of the jets. That creates a richer mixture. First, I cut off a small part of that spring. That made it harder for the piston to pull the metering rods out of the jets. Next, I sanded down the ends of the metering rods so when they DID finally open up, a little more fuel was allowed in. The result was more power when I decided it was necessary to pass someone and an increase in fuel mileage. I noticed it mostly when traveling between home and college because it was a two-hour drive at steady highway speed. That removed the variables of driving around town.
Advancing the ignition timing a little will make the engine develop more power so you won't have to push the gas pedal so far. There also used to be new springs available that were weaker than the original ones in the distributor. That allowed the mechanical timing advance to kick in sooner at lower engine speeds.
Be sure the vacuum hose to the distributor isn't cracked or leaking. One thing that is often overlooked is tight universal joints. Even before you notice a vibration, that binding robs a lot of horsepower. Another trick is to loosen the front wheel bearings just a little. 1/8 turn on the adjuster nuts is sufficient to allow the brake rotors to wobble a little. That won't be felt and it won't be noticed in the brake pedal but it will push the pistons back into the calipers a little. That will reduce drag. Ford has always had real miserable brake caliper mounting designs. This will reduce the drag.
Be sure the preheat tube is intact. That warms the incoming air from around the exhaust manifold and brings it in the bottom of the air cleaner snorkel. Warm air helps the fuel vaporize better. Liquid fuel goes straight out the tail pipe without burning or creating power. Only fuel vapors burn. Check the tube that brings warmed air to the automatic choke. It's common for them to rust off. That makes the choke take longer to open fully. In the late '70s the government mandated that chokes must be fully opened within three minutes of starting the engine. That led to a lot of running problems so some people modified the settings to make the engine run right. If it was over adjusted, the choke might be staying on too long. Also check for an electrical choke heater that is supposed to make it open faster. Many Fords used the stator tap on the generator to power the choke heater. That insured it only operated when the engine was running. If you have that system, use a digital voltmeter to measure the voltage on that wire. It should be around 6 volts when the engine is running.
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