There a few things that can be done to improve the fuel efficiency of older non-computer controlled engines. Surprisingly, ripping the emission controls off from the engine isn't one of them.  Both US and Canadian automotive regulations require all emission controls with which a vehicle was originally equipped to remain in place and removing them is illegal.

Governments have imposed emission regulations on the manufacturers as early as the late 1950's and early 1960's. This required the manufacturers to install emission controls on their vehicles to comply with the regulations. The state of California was one of the first jurisdictions to regulate vehicle emissions and still does so very vigorously. Some of the emission controls have an adverse effect on fuel efficiency but many do not and some even improve it. This page isn't meant to be an exhaustive discussion of emission systems but a brief overview of ones that people often remove for the wrong reasons.

Positive Crankcase Ventilation (PCV) Systems

One of the earliest emission controls was the Positive Crankcase Ventilation system. It has two purposes: the first to prevent emissions from engine blow-by and the second to reduce the formation of sludge in the engine. This device allows a controlled amount of air flow through the intake manifold and removing it will cause the engine to run richer. Although most people aren't likely to remove this device, it is something that is not very well understood and is often not well maintained.

 

Gasoline Evaporation Control Systems

To prevent gasoline fumes from polluting the atmosphere, the fuel tank and carburetor are connected to the charcoal canister. People are quick to remove this device although it has no impact at all on performance and provides a very slight improvement in fuel efficiency. Removing this device may even reduce fuel efficiency because there is a small airflow through the charcoal to recover the gasoline fumes. Without the purging airflow, the carburetor's fuel mixture will become slightly richer. Maintaining this system is as simple as changing the air filter on the bottom the charcoal canister as recommended in your owner's manual.

 

Exhaust Gas Recirculation (EGR) Systems

High peak combustion chamber temperatures cause the formation of various oxides of nitrogen commonly referred to as NOx. The EGR system reintroduces a controlled amount of exhaust gas to reduce the peak combustion temperature. While it is true that this device could have a small negative impact on performance, it can have a significant positive impact on driving costs. Although early EGR systems were crudely controlled, they generally only add exhaust gas to the fuel mixture under load.

An excellent way of improving fuel economy is to optimize the ignition timing.  This often implies running as much ignition advance as possible. Automobile manufacturers keep the ignition point in the combustion cycle later than is optimal for the best fuel economy to keep the combustion temperature low enough to minimize NOx formation. The optimum ignition timing results in the peak combustion chamber pressure occurring at the point in the cycle where the piston can exert the most amount of torque on the crankshaft. Too early and the engine fights against itself. Too late and the power in the cylinder is wasted.

The problem with timing the peak combustion pressure to occur at the optimum point is that the knock-resistance of regular 87 octane is often too low to usually allow this. Engines with a high amount advance will "knock" or "ping" under load. One way of combating this is to use higher octane gasoline. The other way is to let the EGR system do its job of reducing peak combustion temperatures. Under light loaded cruising conditions, very little or no exhaust gas is metered back into the engine. As loads increase (going uphill or passing), the EGR system meters more and more exhaust gas back into the engine. As EGR system often require manifold vacuum to operate, manifold vacuum is very low at full throttle which prevents EGR operation.  While the EGR system may not be able to completely control part-throttle knock, it may allow you to run more ignition advance without requiring you to switch to a higher octane fuel.

Even though the mass of hoses around your engine compartment can look hideous, leave them alone and concentrate instead on optimizing your ignition timing if you want to improve your gas mileage and reduce your driving costs. Make sure that the system is working as it was designed.

 

Fuel Evaporation Systems

There are two separate systems on your engine that work to improve fuel vaporization. The first one is the one that directs thermostatically-controlled warm air into the air cleaner. Second one is commonly known as the heat riser or the manifold heat control.

Drag racers commonly remove the factory air cleaner and replace it with a nice shiny chrome open element air cleaner. While factory air cleaners are more restrictive than the open element ones, which result in slower 1/4 mile times, they are much better for improving gas mileage. This is because there is a thermostat in the air cleaner that tries to maintain a warm airflow into the carburetor and warmer air allows the fuel mixture to obviously evaporate better. Vaporized fuel is more evenly distributed to the individual cylinders and burns more completely in the combustion chamber.

The heat riser or manifold heat control system is designed to maintain a hot-spot under the carburetor. This hot-spot ensures that any fuel not vaporized after the carburetor is vaporized when it hits the manifold floor. When people say that their engine runs better when it is warmed-up, it is the warmed-up intake manifold to which they are unknowingly referring.

These two devices work together and accomplish fuel vaporization in different ways. The thermostatically-controlled air cleaner only functions under cruising conditions. Relatively low cruising air flow to the engine is first warmed by the exhaust manifold hot air stove. After the carburetor, the high manifold vacuum ensures that all fuel is vaporized under cruising conditions.

Under high load (low manifold vacuum), the flapper in the snorkel opens fully and allows only cold air into the engine. With the low manifold vacuum, some fuel remains in the atomized liquid state after the carburetor because there is not enough heat in the air to cause it to vaporize. However, the hot-spot in the intake manifold floor causes any liquid fuel falling out of the airstream and landing on it to evaporate.

If you are drag racing, feel free to disable these systems. If you driving your car on the street and smooth-running fuel efficient vehicle is important to you, keep the fuel evaporation systems in good working order.