Tempest in a Teapot
Insight into one of Holley’s most controversial carburetors
Jim Smart - October 11, 2012 10:00 AM
The Holley 4000 Teapot carburetor with a simple rebuild kit available from Mustangs Etc. Shown here is a 1955 version of the Holley 4000 with a separate automatic choke in the manifold. For ’56, Holley went to an integral automatic choke. The 4000 lived for three model years — 1955-’57. It was used on the supercharged 312 “F” code Fords only for ’57.
The 4000 can easily be viewed as one of the first four-barrel carburetors where normal driving took place on primary bores, with secondaries coming online as needed at wide-open throttle. This was a new approach to four-barrel carburetion in 1955.
Float needle valve and seat look like this and are easy to service without removing your 4000 from your Y-Block engine. Never be careless with small parts over throttle bores, which can easily be ingested, doing grave engine damage.
Secondary connecting tubes carry fuel from the fuel bowl to open secondary throttle bores at wide-open throttle. These tubes are secured at the fuel bowl with O-rings and brass flat washers. You don’t want leakage at these O-rings above the fuel bowl.
The 4000’s accelerator pump is an adjustable, spring-loaded piston type. The accelerator pump sprays raw fuel into the primary bores to compensate for a momentary lean condition between idle and power circuit. You don’t want any kind of lean condition during throttle up.
The air horn is retained with safety-wired screws. They must be safety wired together using .020-inch wire during reassembly to prevent ingestion and engine damage.
Air horn and fuel bowl are also secured via this common cross-slot screw.
This throttle body to main body screw frees up the three main subassemblies.
The Teapot’s throttle body flows 374cfm, which makes it a modest carburetor at best, though adequate for its time. Lincoln Y-block and supercharged Ford Y-blocks got a more generous Teapot at 453cfm.
Traditional Holley types will call this part a power valve. On the 4000, as well as other Ford carburetors of the era, this is a vacuum-operated spark control valve. As we open the throttle, the spark control valve channels a controlled vacuum signal to the distributor’s vacuum advance unit to more precisely control spark advance on Ford Load-O-Matic distributors only. Ford later went to a conventional vacuum advance.
This is the 4000’s power valve, also known as an economizer valve, which richens fuel mixture under hard acceleration hence. Your Holley 4000 carburetor kit will include a new power valve.
There are two idle air mixture screws for primary bores. Gently screw them in to where they just seat, then, back out 1½ turns for start-up and tuning. It is a good idea to put mixture screws back into the same positions.
Be very attentive to tiny parts, like these two secondary check balls, which allow the flow of fuel one way to secondary throttle bores. Fuel flows only at wide-open throttle.
The Teapot sports a traditional brass “horseshoe” float. It must be checked for leakage and replaced as necessary. Float adjustment is straightforward — level with the top of the bowl with approximately 3/8- to ½-inch as shown.
This is the accelerator pump piston in our Mustangs Etc. kit, which appears to be Viton, a durable material for today’s harsh fuels.
Accelerator pump adjustment is crucial to crisp off-idle acceleration and less chance of a lean backfire.
Brass secondary tubes must be seated firmly with fresh O-rings and brass washers peened into place. This is not a spot where you want leakage, especially at wide-open throttle when fuel is flowing through these tubes.
You want clean surfaces and a nice thick carburetor base gasket for best results.
Idle air adjustment can be accomplished with or without a vacuum gauge. At idle, you want the most vacuum possible — above 17 inches. Optimum is 22 inches. One at a time at idle, turn each mixture screw clockwise until the idle speed begins to fall (or vacuum drops), then, slowly back out until idle stabilizes and is smooth. Do the same with the opposite screw. Double check the other until you have a smooth idle.
Mention the word “teapot” around savvy classic Ford guys and you will get a befuddled look because this old time Holley carburetor is a whole ’nother carburetion experience. It is loved and it is surely hated.
The Holley 4000 carburetor — best known for its nickname of “Teapot” — is likely one of the oddest carburetor designs ever, because the fuel bowl is perched atop four concentric throttle bores in a smaller than conventional pattern that ended when the Holley 4150 was introduced in 1957.
The Teapot has a lot of well-deserved nicknames such as “Haystack”, “Coffee Grinder”, and “Firebox” according to the late Jon Enyeart, founder of Pony Carburetors. Although we think of the Holley name for its aftermarket performance demeanor, the Bowling Green, Kentucky-based company has produced millions of original equipment carburetors for the auto industry, including the 4000 downdraft atomizer.
The Holley 4000 was introduced in 1955 atop Ford’s venerable Y-block V-8. This wasn’t the first year Ford put a four-barrel carburetor on top of a V-8. The Holley 2140 four-barrel carburetor preceded the 4000 on 1953-’54 Lincoln sporting a 317ci Y-block V-8, as well as Mercury’s smaller 256ci Y-block V-8 in 1954.
What made the Holley 4000 unique was its concentric four-throat design with vacuum secondaries and a fuel bowl parked on top. Now while this might seem like a slick idea for isolating fuel from engine heat or conceiving a simple design, it created more problems than it solved. Because the fuel bowl is located above four throttle bores, it is vulnerable to backfire, which happens when ignition timing is late or there’s a brief lean condition during acceleration. The backfire ignites fuel in the bowl or any fuel that might be leaking, depending on the severity of the backfire.
Because Jon Enyeart was a very savvy engine tuner and carb guy who knew his business, he offered constructive advice for those with Holley 4000 carburetors. To prevent the unthinkable Teapot carburetor fire, Jon suggested the following in his Ford Carburetor Guide:
• Make sure ignition timing isn’t too late, which is a major cause of backfire.
• Ensure valvetrain is functioning properly.
• Be sure all ignition parts are serviceable. Misfire spark plugs are another culprit.
• Make sure choke system is working properly. (If choke plates are not closing when the engine is cold, you can get a lean mixture backfire.)
• And finally, always carry a Halon fire extinguisher with you, just in case!
The Holley 4000 consists of three major subassemblies — the main body and its cover (fuel bowl), air horn, and the throttle body with four bores. There are four fuel-metering systems in the 4000 carburetor — main metering system, idle circuit, power enrichment system, and the accelerator pump. These four systems work together seamlessly under normal conditions. Keep in mind these systems are in use for normal driving where the secondary throttle package is uninvolved.
When you punch the accelerator, circumstances change considerably, as the secondary throttle plates are activated with a vacuum signal calling on aggressive fuel delivery to the secondary circuit. The thermostatic automatic choke is activated when the engine is cold, closing off air making the mixture rich for cold starting. The choke plates reduce or eliminate hesitation.
Each of the fuel metering circuits gets its fuel from the fuel bowl. Fuel delivery from the pump into the bowl is controlled via the float and needle valve. As fuel is consumed, the float drops allowing more fuel into the bowl. In normal driving, you cannot see fuel flow and float movement. Things become more active when we step on the gas and fuel consumption goes skyward.
The main metering system consists of two main metering jets in the fuel bowl, sized for the application with a variety of jet sizes available. Fuel passes through these jets with the throttle open, to the discharge nozzle where it is mixed with incoming air. The power enrichment circuit consists of a power valve (also called an economizer valve), which is closed when manifold vacuum is high. When the throttle is opened aggressively, vacuum drops and spring pressure opens the valve, allowing more fuel to the main metering circuit. If you could look down the carburetor throat, you’d see a flash flood of atomized fuel at wide-open throttle at the boosters that comes via the economizer valve.
Secondary throttle plates are opened via a strong vacuum signal at the diaphragm when the accelerator is pressed. Check balls allow the flow of fuel one way to the secondary discharge nozzles where it is mixed with inbound air.
The automatic choke is a temperature-affected system with a thermostatic spring, hot air tube, and a fast idle cam to keep idle high during warm-up. As exhaust manifolds warm and air being drawn into the choke stove warms the thermostatic coil, the coil expands, opening the choke plates. As the plates open, the fast idle cam rolls over to normal idle.