Accelerator pumps look different on the various types of carburetors, but their mission is the same: to enrich the mixture as the throttle opens to prevent stalling.
Stumble, hesitation, and flat spots are caused when the accelerator doesn't provide enough of a fuel shot as the throttle is opened. This accelerator pump and nozzle is what you can expect to see on Holley and Barry Grant carburetors.
This is the idle-mixture adjustment on an Autolite 4100 four-barrel. With the throttle at idle, fuel and air mix here to keep the engine idling. When adjusting idle mixture, you are adjusting the air/fuel mixture. If the engine surges at idle, one side is too lean. A rough idle indicates a mixture that's too rich. At idle, the smaller transition hole should be covered by the throttle plate.
This is the transition slot that delivers fuel as the throttle is opened to provide a smooth transition from the idle circuit to the power circuit.
Secondary throttle plates remain closed until we mash the gas. At that time, a strong vacuum signal in the primary bores opens the secondary throttle plates. Of course, on carburetors with mechanical secondaries, the secondaries open via linkage. In either case, as the secondaries open, fuel is drawn from the secondary fuel bowl via the secondary boosters.
Doctor of Tune
Jon tells us the best tuning comes from putting an engine through its paces on a chassis or engine dynamometer. On a chassis dyno, the engine is run through the driveline under load. Fuel mixture, rpm, exhaust-gas temperature, and more, are an indication of what the engine is doing as it operates. The proper air/fuel mixture can be determined with a tailpipe sniffer that measures oxygen content, NOX, and hydrocarbon emissions. All are important to proper tuning. We can also determine volumetric efficiency (VE), which helps determine carburetor and manifold sizing for the application.
It's easy to assume that dyno testing only determines power at wide-open throttle, but it can also indicate how the engine behaves at cruise power. It's good to know what the engine does at 70 mph as well as during deceleration and idle.
Air/fuel mixture is primarily determined by jet and throttle-bore size. Main metering jets in the carburetor control fuel flow to the boosters in the carburetor throttle bores. Boosters mix the fuel and air. The larger the jet size, the more fuel reaches the boosters. If the mixture is too rich, reduce the jet size to reduce fuel flow to the boosters. By the same token, if the mixture is too lean, a larger jet size is needed for greater fuel flow to the boosters.
Boosters and main metering jets do their work when the throttles are open, and there's airflow through the boosters. When the throttle plates are closed and the engine is at idle, fuel flows through the idle circuit and atomizes below the throttle plates. As the throttle opens, fuel moves from the idle circuit to the transition circuit, which coupled with the accelerator pump shot, helps the carburetor move from idle circuit to power circuit. One objective for tuning is to help the engine make a smooth transition from idle to power making. Clean fuel and air circuits are required, coupled with a good accelerator-pump shot as the throttles open.
Proper air and fuel distribution is needed under all conditions. If you step on the throttle and the engine stumbles, this indicates a weakness at the accelerator pump or transition circuit. This is when you want to examine accelerator-pump shot. Does raw fuel spray from the accelerator-pump nozzles? If it doesn't, the accelerator pump is at fault. If there is fuel spray, the problem could be either an insufficient amount of fuel or dirt, irregular passages, and so on in the transition circuit.
Inspect the choke, which provides a richer mixture when the engine is cold to prevent stalling by reducing air supply. At the same time, a fast-idle cam holds the throttle open a pinch more to induce a faster idle.
Automatic chokes operate based on engine temperature. As the engine warms, the choke opens up, leaning the mixture. Cold stalling is an indication of lean choke and overly rich conditions if your engine is belching a lot of thick, black smoke from the tailpipe and idling rough. Most of the time, chokes don't come on enough, if at all, which causes a lean condition when cold as indicated by stalling.
The choke should come on approximately 3o4-closed for a good cold start. Every engine mandates something different that requires choke adjustment. Some need more choke than others. Much depends on ambient temperature and engine temperament, e.g., cam profile, carburetor, and so on.
Sometimes you do all you can for a carburetor, and the engine still won't run properly. Ford 170 and 200ci sixes, for example, suffer from poor idle quality and lackluster performance. The long intake manifold cast into the cylinder head is the main reason these engines struggle. Fuel distribution is poor because of the long, roughcast internals that interfere with air/fuel flow. Cylinders 1 and 6 get shortchanged most of the time, followed by cylinders 2 and 5.