Use a filter to avoid carburetion...
Use a filter to avoid carburetion problems to begin with. An inline unit like this one from Barry Grant is a good way to stay out of trouble.
Before anything else, be certain that a clean and filtered supply of fuel is provided. Contaminated or dirty fuel is one of the largest causes of improper carburetion. When installing a carburetor, renew the fuel filters. Place one directly before the fuel pump and the other before the carburetor.
Float Bowl Facts
After a rebuild or before attempting any other adjustments, checking and adjusting the float level comes first. This should be the first thing you do before attempting to make any carburetor adjustments. The float functions like the float in a toilet tank and shuts off the flow of incoming fuel entering the bowl by closing a needle on the seat of the fuel inlet. The float level should put the fuel level just below the bottom of the sight plug hole if your carburetor is equipped with one. You can make the adjustment with the vehicle on a level surface and the engine idling.
The BG inline filter uses...
The BG inline filter uses a replaceable cartage filter for a first class filtering job. Most new carburetors come adjusted to run out of the box and the way we get into trouble is by overlooking the obvious things like clean fuel.
This cut away diagram shows...
This cut away diagram shows how the float mechanism works. When the fuel level in the bowl rises to the correct level, the float closes off the needle against the seat. Incorrect float level can be at the root of several different problems, including poor idle and engine flooding.
First, remove the sight plug....
First, remove the sight plug. We had a shop towel at the ready for any fuel spillage. If your carb is equipped with a clear sight plug you won't need to do this and will be able to sight the fuel inside the bowl with a flashlight.
To make your adjustment you...
To make your adjustment you will need to loosen the lock nut on the needle and seat. This will allow you to turn the adjusting nut to raise or lower the float level. Each hex flat on the nut will change the float level approximately 1/32-inch. When you have the fuel level just below the sight plug hole you will then tighten the lock screw and reinstall the sight plug. Make sure you have a shop towel handy in case you have any fuel leaks from the sight plug or needle and seat adjusting nut.
If you'd rather perform an...
If you'd rather perform an exact measurement or your carb isn't equipped with a sight glass or hole you can remove the float bowl from the carb as shown. If you carb is on the engine you'll need to drain the bowl first.
Once the bowl has been removed,...
Once the bowl has been removed, the float can be set to factory specifications using any precise measuring device. Repeat the procedure for both primary and secondary float bowls. With the bowl held upside down, the distance to the bowl ceiling is measured and set at the desired distance.
This photo shows what your...
This photo shows what your accelerator pump shot should look like. If you don't see fuel discharged into the carb like this, then look for problems with the accelerator pump. The fuel is shown squirting out of the accelerator pump nozzles, which can be changed in size to control fuel delivery speed.
The most common cause of a stumble off-idle or bogging is an inadequate or incorrect accelerator pump shot. The first thing to do is to look at the discharge nozzles and make sure you are getting a good, strong shot of fuel when you pull the accelerator linkage back.
If there is no shot of fuel then you need to inspect the pump diaphragm for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. The accelerator pump system consists of three main components: the pump diaphragm, the pump cam, and the pump nozzle. This is the system on your carburetor that is most responsible for having good, crisp, off-idle throttle response. Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened. By doing this, it acts to smooth the transition between the idle and the main operating circuits so that no stumble, hesitation, or sluggishness will be evident during this transition phase.
Very often the source of the...
Very often the source of the problem is a perforated or otherwise damaged accelerator pump diaphragm. Replace the diaphragm with a new unit. This collection of accelerator pump parts shows the diaphragm in the lower right corner of the photo.
Once this clearance has been set, make a careful inspection of the pump linkage and work the throttle. Make sure that the accelerator pump arm is being activated the moment that the throttle begins to move. This will assure that pump response will be instantaneous to the movement of the throttle. These adjustments can be made by turning the accelerator pump adjusting screw that is located on the accelerator pump arm together with the pump override spring and lock nut. The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for a pre-determined amount of fuel to be delivered is affected by the pump nozzle size. A larger pump nozzle will allow fuel to be delivered sooner than a smaller pump nozzle. During acceleration tests, if you notice that the car hesitates at first and then picks up, it's a sign that the pump nozzle size should be increased. A backfire or lean condition upon acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size may be called for.
Throttle at wide open application often calls attention to secondary opening issues. To see if your vacuum-operated secondary is opening, its operation can be checked using a paper clip. The engine needs to be under a load before they will open. Take a normal paperclip and clip it onto the secondary diaphragm rod. Then push it up against the bottom of the secondary diaphragm housing and go out and drive the vehicle including a pass at WOT. When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod, then you can tell that the secondary has opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.
With a good diaphragm in place...
With a good diaphragm in place the first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover's arm, at wide open throttle. This clearance should be around 0.015-inch. The purpose for this clearance is to assure that the pump diaphragm is not stretched past its maximum limit at wide open throttle, which would cause premature pump failure.
Secondary Questions
This...
Secondary Questions
This Holley 4160 carb is a popular choice for a mild performance car with an automatic transmission. It has a single feed fuel line with no secondary metering block and a vacuum operated secondary. The black secondary actuator is seen on the side of the carburetor body and it contains not only a vacuum diaphragm but a calibrated spring that controls secondary engagement speed.
To change secondary opening...
To change secondary opening rates, different calibration springs can be interchanged. Variables in secondary spring selection will include vehicle weight and transmission type. For street cars, the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a mechanical secondary (called a double pumper) because they work by sensing engine load. This mechanical secondary carburetor is best on a lighter car with a more radical camshaft. In addition to the mechanical secondary actuation the rear bowl also has another accelerator pump to compensate for the immediate inrush of air that results when the secondary snaps open. This is what is meant by the term "double pumper." A lower axle gear ratio and manual transmission work best with a carb of this type.