Why Is My Idle Rough?
- Idle mixture screws are bent, or scored.
- Idle mixture is out of adjustment.
- Idle passage(s) may be plugged. Look here for where the passages are.
- Vacuum leak – spray carb cleaner around mounting, intake manifold and any vacuum line being used. Mounting may be warped, or you might hear a whistling noise from the leak.
- Look down the carburetor at idle and see if gas is dribbling down the venturi. This should not be happening and means the gas is being siphoned into the bore. That could be a check ball not seating (light tap on the check ball will seat it). Check weight on top of check ball may be missing. Gas cap not vented. Float bowl vent plugged off causing a vacuum effect in the float bowl.
- Throttle bore channels are dirty, or carbon build up.
Dies as if Running Out of Gas
Here is a list of possible causes not in any order.
- If you can partially close the choke and it keeps running then you proved it’s not enough gas, or maybe a vacuum leak.
- For vacuum leak use carb cleaner and spray around mounting, intake manifold and any vacuum lines you may have. If the RPM picks up you found the problem.
- Weak fuel pump. Test the pump and compare with your motors manual specs, but should be around 4-4.5 lbs.
- Float valve sticking closed. Did you installed the spring that hooks the needle with the float. This is used to pull the needle out.
- Kinked gas line, or plugged gas line.
- Plugged fuel filter.
- Float level.
- Float sticking. Move the float up and down and feel for any catching. The float pin might be worn.
- Plugged main jet passage. Spray carb cleaner into the main jets to see if it goes all the way through. This may help dislodge any crud in the passage.
Gas Pedal Sticks
- Throttle linkage might be binding somewhere. Disconnect from carburetor to see if that eases the binding.
- The return spring in the accelerator pump diaphragm might be installed on the wrong side of the diaphragm, or it might be broken. See Accelerator Pump information.
- The diaphragm could be dried out and stiff.
- The diaphragm housing lever could be binding, or is too loose to operate correctly.
Whistling Coming From Carburetor
This is most likely a vacuum leak. Look at the mounting and possibly the power valve might be leaking.
Runs Out of Power at Higher Speeds
- Check the float level. Use the specification sheet that came with your carburetor kit. The setting is different depending on engine size and make. The float level could be a bit low. Don’t cheat and just move it to get more gas. You will cover up something else.
- Test the fuel pump. It may be getting weak. Be careful when buying a new pump. They often put out too much pressure. 4 to 4.5lbs would be OK. Check your motors manual to be sure.
- Any dirt, or corrosion in a passage will cause this.
- Check your spark plugs for color. If they are burning white then your jets may be too small. Increase .002 at a time until you get tan, or light brown.
- The carburetor might be a transplant and is wrong for the engine. The venturi, or bore size may be too small.
Electric Choke Conversion Opens Choke Too Quick.
- The thermostat may not be putting enough tension on the choke valve. With the throttle open a bit, twist the choke until the choke valve closes, then give it another 1/8 turn. Make sure it takes a bit of finger pressure to open the valve.
- Do not connect the choke to the + side of the coil. There is usually a resistor between the ignition and coil.
- Connect to a source that is hot only with the key on. Examples: Stator of an alternator, or hot side of the windshield wiper.
- When installing the rheostat there needs to be a clean contact where it mounts.
- Leaving the key on for a prolonged time, engine off can damage the electric choke along with your points and other ignition parts.
- Disconnect any choke pull-off as a test to make sure they are not opening the choke valve prematurely.
Why Engine Races When Started?
- Throttle valves might be in wrong cause one or both to stay open, Be sure they both close completely with throttle closed and not on fast idle.
- Vacuum leak – This would probably accompany a rough engine.
- Stuck throttle linkage. Disconnect and try again.
Why Does My Engine Hesitate?
- Distributor advance not working correctly. Look below the points to see if the springs are OK.
- Accelerator pump circuit not squirting enough gas. Look down the carburetor with engine off. Pump the gas once. You should see 2 good squirts coming out of the venturi. If not then you have pump problems. Could be the check ball isn’t seating, Pump diaphragm spring is wrong. Diaphragm leaking. Clogged passage from diaphragm to main discharge. Check valve not working.
Hesitates or Bogs
- With engine off look down the carburetor and pump the gas once. You should see 2 strong squirts coming out of the venturi.
- The inlet or outlet check ball may not be sealing, or someone put in the wrong size.
- Test the accelerator pump system by putting fluid in the float bowl, we use mineral spirits, and pulling up on the accelerator pump to see if the pump well is filling up.
- Press down on the pump and fluid should come out of the main discharge.
- Hold down the check ball in the main discharge an push down on the pump. You should feel a slight pressure. If not, then tap lightly on the check ball to form a better seal.
Why Does My Carburetor Backfire?
This is caused most likely by a very lean condition. Either there is a vacuum leak, causing too much air, or there isn’t enough fuel getting to the intake manifold.
Check the carburetor mounting gasket, where the throttle body connects to the float bowl, vacuum hoses and fittings and the intake manifold.
Hard starting after turning the engine off.
Number one is ignition problems.
Look down the carburetor after turning the engine off. Gas should not be dribbling out of the venturi, or main discharge. If it is then the check ball may not be sealing, or because of bad venting the float bowl has pressure, forcing gas out of the discharge.
- Power valve bad
- Float level is off
- Fuel filter is dirty
- Fuel hose is kinked, or if a rubber hose may have been eaten by ethanol and will collapse inside.
Vacuum channels, or power restriction are plugged, carboned up, or some other obstruction.
Why Does Gas Leak Out of the Gaskets?
We don’t have any problems with our gaskets but there are other considerations:
If you happened to rebuild your carburetor with an old NOS kit then the gaskets may just be too old.
WD40 around a carburetor will destroy gaskets and cause them to leak.
A warped carburetor can cause gas to leak out past the gasket.
If the carburetor is flooding, then gas will leak anywhere it can find an opening.