Is my carburetor at fault?

October 1, 2017 | By Staff

Question:

I have a 1960 Chevrolet with a later 350 small block with a Rochester Quadrajet four-barrel. It’s hard to start and coughs and hesitates when I really step on it. My neighbor seems to think it might be problems with the carburetor. We tried adjusting the idle mixture screws but that doesn’t seem to make a difference. Should I go for a rebuild or bite the bullet and just buy a new one?

Answer:

The first question I would ask is what is the airflow requirement of your engine? If it is a stock 350 small block and you have a big 750 cfm (cubic feet per minute) carb on it, your engine is way over-carbureted and will have a mushy mid-range response, among other problems. Overcarburetion is a very common problem on engines that have been tinkered with. Most likely your engine only needs between 450 and 550 cfm to run well. (A bigger carb may be justified if someone has done some extreme tuning on the engine such as special heads, bigger valves, hot cam, etc.)

If your engine has the right carb, I would say next you should check out your ignition system. Many mechanics make the same mistake of fiddling with the idle mixture screws and changing the idle speed on the carburetor when trying to remedy a rough-running engine. I suppose they do this because the carb appears sort of user friendly, though engine problems are almost never the fault of the carburetor mixture and idle speed.

Because carburetors are generally reliable, before you tear yours down, make sure your problem doesn’t lie elsewhere.

See that your engine is in proper tune and its timing is right. If the engine is equipped with a breakerpoint distributor and it isn’t running right, it’s usually due to ignition problems. Only after you’ve eliminated ignition as the source of your troubles should you check out the fuel system.

One problem common to classic-era carbs equipped with automatic chokes is sticking linkage. Clean all of the choke linkage with lacquer thinner or solvent to free it up. DO NOT oil it. Oil will burn and cause sticky varnish at points of movement and increase your problems in short order.

If your car stumbles as if it is out of gas while cruising, but then catches and goes on, you may have a clogged filter, a bad fuel pump, a clogged fuel line or dirt or water in the jets and float bowl of your carburetor. However, if your engine is equipped with a modern in-line fuel filter as yours should be, the problem is more likely to be a clogged filter or bad fuel pump.

To check for a bad fuel pump, take the rotor out of the distributor to cut down on the risk of fire, then disconnect the fuel line at the carburetor. Hold a metal catch vessel in front of the line, and then have a friend crank the engine. If no fuel comes out, you either have a bad fuel pump or clogged fuel line.

If you get a little flow of fuel, hold your thumb over the end of the line and crank again. There should be perceptible pressure against your thumb. If there is not, try blowing out the fuel lines, then inspecting them for kinks. If the fuel lines are clear but you are still not getting pressure, you probably need to rebuild your fuel pump.

If your ignition system, fuel filter, fuel pump and fuel lines are as they should be, the carburetor is probably at fault. Remove the carburetor after running the engine for a short time so the float chamber will be full of fuel. That way you can more easily determine if your gasoline is contaminated and if the float is functioning properly.

Place the carb on your bench, and then carefully remove the float bowl cover. Now slide out the hinge pin for the float. Lift the float out and inspect it for leaks, and then examine the contents of the float chamber. If you find dirt, water, rust or scale, it’s time to overhaul your carb.

If your carb spits and coughs and you hear a popping in its throat and your engine runs hot, your carburetor has dirt in its jets or has an air leak due to defective gaskets where the carburetor attaches to the intake manifold, or where the manifold bolts to the block. Use a vacuum gauge to find manifold leaks.

These symptoms may be caused by dirt in the jets, a stuck needle valve or a float that is adjusted too low in the reservoir. The problem could also be a hole in the float, causing it to sink too low in the float chamber. To fix these problems, remove the carburetor, disassemble it, clean it and replace any worn parts. Finally, if you suspect that the idle mixture is incorrect, try adjusting the screws together, evenly a half-turn at a time, making sure you keep track of how far you’ve adjusted them.

If your car is pumping black smoke, flooding and stinking of gasoline, your engine is running too rich. Inspect the air filter. If it checks out, inspect the choke as outlined above. It could be stuck in a closed position. If the choke checks out OK, the problem may be a sticking needle valve in the float chamber of the carb. However, if your carb leaks fuel down its sides, the problem is most likely a stuck float or faulty needle valve.

Rebuilt and new Rochesters are available from auto supply stores, Summit Racing Equipment and speed shops everywhere. And rebuilding services for carbs are in the yellow pages of your phone directory if you don’t want to tackle the problem yourself.

You can purchase rebuild kits and new carbs at: