Troubleshooting a Chevy Carbureted Engineby Don Bowman
Carburetors are troublefree as long as they remain clean, and a fuel filter is used. For the most-efficient fuel mixture on a Chevy carbureted engine under different temperature conditions, a heat insulator block should be used under the carburetor. This keeps the engine's heat from being transferred to the carburetor. Cooler fuel is denser.
If the vehicle runs well but has a bad dead spot when accelerating (it acts like it runs out of fuel for a second or so before it goes), the accelerator pump is malfunctioning. Fuel is withdrawn from a carburetor by vacuum from the engine. When the gas pedal is suddenly depressed for acceleration, the throttle plates open and momentarily the vacuum in the engine goes to next to zero. No fuel can be drawn into the engine on initial acceleration, so the accelerator pump within the carburetor pumps a predetermined amount of fuel into the intake to make up for the sudden drop in vacuum. By the time the engine has burnt this fuel, the engine has once again brought the vacuum back up to 18 inches.
If the vehicle runs rough at an idle and fuel can be smelled, the float bowl is overrunning. The float in the float bowl works like a toilet bowl float. The carburetor is flooding the engine, and the carburetor should be cleaned. The needle and seat in the float bowl should be replaced. If the vehicle is hard to start in the morning and then runs fine the rest of the day, the choke is not functioning properly and needs repair. If the engine starts fine when it is cold but will not stay idling until it is warm or does not run at about 1,200 to 1,500 RPM when first started, the fast idle is out of adjustment. If the engine lacks power and a popping can sometimes be heard in the exhaust along with a pinging in the engine when accelerating, the carburetor is dirty and needs rebuilding. The popping sound and lack of power occurs because the carburetor is running too lean---there is not enough gas and to much air. If the engine runs well but black smoke can be seen when accelerating and the plugs are black when inspected, the carburetor is too rich--too much gas and not enough air. This requires cleaning the carburetor. If the engine is rough just at idle, then the idle mixture jets in the front of the carburetor need to be adjusted.