ENGINE DOESN'T IDLE
This assumes the electrical system is working properly. Most idle problems are due to electrical issues so don't assume it's the carburetor.
- Make sure the idle passage is open. Use nylon brushes to clean ethanol deposits if needed
- Check that the idle discharge holes are clear
- Make sure the passage from the air bleed to the adjusting needle is clear
- Check for vacuum leaks
- Are the throttle plates installed correctly with the taper going towards the top of the carburetor. If installed wrong the throttle plate will allow an air leak into the carb and it will act like it's under throttle
FLOODING OR OVERFILLING
Do you have fuel coming out the top of the carburetor or around the throttle shaft?
NOTE: Fuel should never come in contact with the throttle shaft. If you have fuel leaking around the shaft it's because the carburetor is overfilling, fuel is running down the throat of the carburetor and then our around the shaft which is the point of least resistance.
NOTE: the float level is often blamed for flooding, this is rarely the issue. A float would have to be severly out of spec to allow enough excess fuel into the bowl to cause flooding.
- Test the needle and seat for leaks
- Test the float - brass floats can be tested by submerging the float in hot water and looking for bubbles. Nitrophyl floats can only be tested by weight and should be replaced during your rebuild
- Move the float up and down, look for catching or rubbing. The float pin may be worn
- Test the fuel pump pressure. Single barrel carburetors require 3.5-4psi. If your fuel pump is overpressurized, install an in-line pressure regulator
ONLY RUNS WITH CHOKE ON
This means you have too much air, or not enough fuel. Possible causes:
- fuel filter is clogged
- Fuel line kinked
- Fuel pump weak
- Carburetor has been coated with varnish which can result from sitting too long
- Passages clogged. Spray carb cleaner through each passage and use a nylon brush to clear residue
- Float is out of adjustment
- Vacuum leak
ENGINE POPS WITH STARTING
Most likely caused by a lean fuel condition
- 1st look down the carburetor with the engine off, pump the throttle once. You should see a squirt of gas going into the bore. The accelerator circuit should provide an extra squirt of gas when starting cold. The engine needs this to start. If you don't see that squirt of fuel you may have a bad pump or blocked pump circuit.
- Then check for reasons the gas is low in the float bowl
- Sticking needle and seat
- Float out of adjustment
- Weak fuel pump
- Clogged fuel filter
- Kink in the fuel line
Question
Answer
I have a 59 GMC pickup that has a 270ci with a Zenith 28/228 (228BV11) carb on it. I seem to have air screw holes on both the carb and the throttle body. The throttle body had a screw in it when I got it, and the hole on the carb was wide open. I watched your videos on youtube and you mention that a 28 should have the screw on the carb and the 228 should have it on the throttle body. What do I need to do to correct this?
Answer
Assuming someone hasn't put a 28 top on a 228 bottom, you simply plug the top hole. Use a carburetor clean out plug and if you don't have one of those, use a small amount of JB Weld.
