Jack up all 4 wheels, one at a time or all together, it makes no difference, make sure all the wheels spin freely. Check for vacuum leaks, wave an unlit propane torch around the engine to see if the engine speeds up. Make sure your thermostat is working properly, air up your tires. If you use left foot braking, make sure you don’t rest your foot on the pedal. Don’t drive like you are mad at everybody else on the road.
These are all after you check out what ok4450 told you.
This truck originally had throttle body injection. If you’re using the original fuel pump setup, it’s too much pressure. 9-13 psi is fuel specs for a throttle body and i think a Q-jet uses around 7.
So the carb is geting to much gas
Specs for carb are 4-6.5 psi. So yes fuel pressure is too high for carb.
Too high fuel pressure could certainly dump too much gas into the engine, causing poor gas mileage and poor driveability. This is one of the many potential perils of installing a carbureted engine where a fuel injected engine used to live.
As an added note, others have said that, oftentimes, a four barrel carburetor will get better fuel economy than a two barrel, provided you keep your foot out of it. This has been my experience as well. My father once owned a 1971 Chrysler Town and Country wagon with a 400 big block in it. It had a two barrel carburetor on it when he bought it and averaged 10 miles per gallon. Installing an 850 cfm Carter ThermoQuad in it’s place brought the mpg’s to 15 mpg, and the car had better get up and go all around (though still not spectacular since the 400 was a real dog).
The part about excessive fuel pressure is an excellent point as excess pressure will sink the float in the carburetor.
What I don’t get is if this is the case then how would the truck be driveable, much less to the point of being able to ascertain fuel mileage?