There's much to be said for technology-it certainly has had an impact on the performance of new cars and trucks. But there's also a fair amount to be said for simplicity. Just because something is simple doesn't mean it's not effective. That certainly holds true for carburetors. While electronic fuel injection relies on a variety of sensors and a computer to deliver the appropriate amount of fuel to the engine, carburetors simply rely on vacuum. Basically, all carburetors-single-, dual-, or four-barrel-work the same way and have five distinct systems.
Float
The float maintains a constant level of fuel in the float bowl, and while it seems to be a simple system, it has to function correctly to maintain the proper air/fuel ratio. If the fuel level is too high, the engine will run rich and may even flood. Too low and the engine may run lean.
Main System
When the engine is running, it draws air down through a restricted area called a venturi, and a low-pressure area is created while the fuel in the float chamber is exposed to atmospheric pressure by a vent. As there is more pressure in the float chamber than the venturi, fuel flows through a restriction called a jet and is discharged into the airstream. The airflow through the venturi is controlled by the position of the throttle valves (as well as engine speed); the maximum amount of air the carburetor can pass is its cfm (cubic feet per minute) rating. But while all carburetors are rated in cfm, comparisons can be difficult because the standards for testing are different. Four-barrels are tested at 1 1/2 inches of mercury; single- and two-barrels are tested at 3 inches of mercury.
Idle System
At idle and very low speeds the throttle valves are not open far enough to create a vacuum in the venturi. As a result, no fuel is discharged. However, there is vacuum below the throttle valves, so to supply the fuel the engine needs at idle, there are fuel passages below the throttle valves called idle ports. The idle mixture screws control the amount of fuel that is discharged. As the throttle valves are opened, the idle ports are no longer exposed to vacuum, and the fuel supply stops.
As a part of the idle system, most carburetors also have transfer ports, small slots located just above the idle ports that supply fuel as the transition is made from the idle to the main system.
Enrichment System
When an engine is under a heavy load, a richer air/fuel ratio is needed to produce maximum power, so most carburetors have some sort of enrichment system. some carburetors use a power valve, which is basically an extra jet that opens to supply more fuel when necessary, or tapered metering rods that fit into the jets and can vary the size of the orifice(s) as necessary. Enrichment systems may be mechanically or vacuum controlled.
Accelerator Pump
If you've ever looked down a carburetor as the throttle was opened (hopefully when the engine wasn't running), you probably saw a squirt of fuel being discharged into the venturi. When the throttle is suddenly opened, the air accelerates and in effect leaves the fuel behind for an instant. As a result, the engine goes lean and stumbles. The accelerator pump's squirt prevents that from happening.
Long Live The Holley 94
Often mistaken for strombergs, Holley 94s were used by Ford from 1938-57, which means there were a bunch of them made, and many of them are on classic Ford pickups. The lineage of these carburetors is somewhat convoluted. Henry Ford was looking for a more efficient carburetor than the strombergs his company had used since 1934, so the Chandler-groves Company was enlisted to develop an entirely new carburetor. once it was finished, they were given a one-year contract to supply all the carburetors for Ford's '38 production run. In exchange for that agreement, Ford was given the patent rights on the new design, and when the year was up Henry took the blueprints and went looking for a better price on carburetors. With some minor changes to the design, Holley cut the price by 10 cents apiece and became the sole supplier of 94s until production came to a halt in 1957. While the '38 carburetors are labeled Chandler-groves, Holley made many with the Ford script on the float bowl. some later-model carburetors have a 94 cast into the bowl; they are usually Holley model r-713s or 2100s sold by Holley as replacement carburetors.
Although the 94s and 97s share the same three-bolt mounting pattern and look similar, there are a number of significant differences between the two designs. on the 94s, the fuel inlet is in the top of the float bowl rather than the side of the bowl as on 97s; 94s use a centerhung float compared to the side-hung 97; 94s use spray bars to discharge fuel in the main system; 97s use emulsion tubes. All Holley 94s aren't created equal, either- the original 94 had 0.94-inch venturii, and later y-block versions had 1.0 and 1.062 venturi with a 1 or 1 1/16 cast in the side; cfm ratings range from 155 for the smallest versions to 185 for the largest. After tallying all the variations, when production ended there were at least 17 versions of the 94.
Rebuilding The 94
In most cases, rebuilding a carburetor primarily consists of a thorough cleaning and replacement of the gaskets, needle, and seat, and in the case of a 94, the power valve. But when working with parts as old and with as many miles as most of these carburetors, some additional parts may be needed. on high-mileage carburetors it's not unusual for the throttle shafts or the holes in the carburetor base to be worn. new standard-size shafts, oversize shafts used to save a worn base casting, as well as extended shafts to allow the use of a linkage for multiple carburetors are all readily available.
Like the modern versions, Holley 94s use a power valve (also called an economizer valve) that opens to supply extra fuel to the engine when the vacuum drops to a certain point, usually 7 1/2 inches Hg or less. While they work fine in stock applications, if a more aggressive- than-stock cam is used or multiple carburetors are installed, the vacuum signal is altered and the power valve(s) open prematurely, making the mixture much richer than necessary. A common mistake in this situation is to rejet the carburetor(s), which results in even worse driveability problems. Another mistake often made is to plug the power valves, something that's not recommend. The best approach to this problem is to stick with stock jet sizes (or very close) and use power valves that open at a lower value. In the past, we've used 3 1/2-, 4 1/2-, and 5 1/2-inch Hg valves, depending on the cam used, with excellent results.
Selecting the proper power valve can be done a number of ways. The best method is to use a portable exhaust gas analyzer to check the air/fuel ratios under real-world conditions and try different power valves until you find the right combination. Another way is to use a vacuum gauge. drive the car and note the vacuum readings at idle, then install power valves with opening points in between the vacuum level at cruise and wide open. Then there's the seat-of-thepants method. If the car stumbles and puffs black smoke when you jump on it, the power valves are opening too soon, so go to a lower-number valve. If it pings, do the opposite and try a higher number. Finally, there's the best guess method. If you're using two carbs, go about half of the stock value of 7 1/2 inches of vacuum Hg, which is in the 3 1/2- to 4 1/2-inch Hg range.
While swapping power valves is a simple cure for an overly rich mixture, we should point out that while contemporary Holley power valves will fit in a 94, they don't seal properly. The early carbs have a smaller sealing surface than subsequent designs, and the later power valves have a radius at the bottom of the threads that prevents a good seal. However, Charlie Price at vintage speed stocks a wide selection of modified power valves that fit and seal like they should. (He also has virtually every other part you might need for 94s.)
When F-100 aficionado Paul Willis decided to build a pair of 94s for a Flathead, he dug through his stash of y-block parts, called in a few favors from buddies, and soon had a box full of 94s for next to nothing. As there were at least 17 versions of the Holley 94 produced, the first challenge was to find a matched pair- the second was to find a base casting that didn't have worn-out throttle bores. He lucked out on both counts. With a matched pair of rebuildable 94s on hand, Paul contacted speedway Motors and ordered two rebuild kits and two extended throttle shafts. In addition, he took an educated guess and had Charlie Price of vintage speed send a couple of pairs of power valves, 3.5- and 4.5-inch Hg. Once the parts were on hand, we went about freshening up a pair of Holley 94s. Here's how you can do it too.
The New Stromberg 97
Certainly one of the most popular vintage two-barrel carburetors is the stromberg 97. designed over 70 years ago, the stromberg family of carburetors included the type 40, 48, and 97 used on the '35 through early '38 Ford 85hp engines. In addition to the carburetors manufactured by stromberg, the Bendix Corporation built replacement 97 carburetors under license. now brand-new 97s are available again. After years of rumors about their existence, they're a reality. Check with the manufacturer or u.s. dealers such as H+H for more information.
