Modified Mustangs & FordsHow To Engine
Ford Mustang Carburetor Tuning Technology
Take The Mystery Out Of Carb Tuning
Use Filtered Fuel
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.
Accelerator Pump Issues
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.
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.