What could be the causes of high fuel consumption on carburated engines

Anything that causes a poor state of tune, literally anything and everything needed for the engine to operate. Mixture to timing.

1. Go over the whole ignition system. Replace, for example, spark plugs (with exact brand and plug number that came in the vehicle), spark plug wires, rotor, and distributor cap. Replace platinum plugs at half the recommended mileage. If it's an older engine, replace the points and condenser.

2. Check idle rpm (tachometer) and engine timing (timing light).

3. Replace air filter and fuel filter. A new air filter alone can improve gas mileage by as much as 10%.

4. Check the Thermostat. If the incorrect thermostat is installed, or it sticks open, gas mileage can be radically affected.

5. Check Tire Pressure. You can lose up to 6% of fuel efficiency for every pound per square inch (psi) below the manufacturer's minimum recommendation.

6. Add Shell Regane or Chevron Techron fuel injector cleaner to gas tank every 6 months. This will clean valves and combustion chamber.

7. Use full-synthetic lubricants; e.g. engine oil, manual synchromesh transmission fluid, automatic transmission fluid, power steering fluid, differential fluid, and wheel bearing grease. This should easily increase gas mileage 8 to 10%.

8. Check for dragging brakes, including emergency brake.

9. Replace faulty gas cap.

10. Remove excess weight. Remove unused luggage, bicycle or ski racks and other extra weight you may have in your car. Every 200 pounds of unnecessary weight reduces mileage by one mile per gallon.

11. Check alignment.

12. Turn off the air conditioner whenever you can. Running your A/C causes extra work for the engine, which lowers fuel efficiency by 5-10%.

13. Reduce speed--don't exceed 60 mph (96 kph). On the highway, more than 50 percent of the energy required to keep rolling is aimed at overcoming aerodynamic drag. As the pedal hits the metal, the aerodynamic drag and rolling resistance increase. That's why the 55-mph speed limit was clamped on the highways in the '70s. Above 55 mph, fuel economy takes a dive. So, by cutting highway cruising speeds to 65 mph instead of 70 mph, gas is conserved. When you use overdrive gearing, the engine speed decreases, reducing both fuel consumption and engine wear. On the open highway, cruise control maintains a constant speed and can also help reduce fuel consumption.

I like #5 best of all the list. The most overlooked, and easiest to correct.