It's impossible to write one article that addresses how to rebuild/restore all types of carburetors, because the history of the automobile is rife with nearly endless approaches to their design. Eventually, we will archive specific articles on how to rebuild and troubleshoot each type of carburetor — at least the most popular ones — but for now we'll concentrate on one common example, the Holley two-venturi, or two-barrel.
Why the Holley and not a Carter or Rochester? Well, because the Holley was fitted to a staggering number of 50s and 60s cars, and the majority of collectors these days seem to be concentrating on these two decades (that, in turn, makes sense because the typical collector is middle-aged, making the 50s and 60s the era with which they are most familiar). Another reason is because the overwhelming majority of cars of that era had V8 engines fitted with two-barrel carburetors, many of which were made by Holley or under license to that company. We chose a two-barrel carburetor because it is relatively simple, while still containing all five basic "systems."
This particular carburetor is fitted to a 1964 Ford Galaxie XL powered by a high-performance 390 cubic inch engine with three two-barrel carburetors. In this configuration, the center carburetor is a standard unit that would have been fitted to any production engine. The front and rear carbs have no power valves, since they only operate at idle and partial/full throttle.
Don't start until you re-read our carburetor theory article. Before working on one you must be familiar with how it works, so take the time now to get a thorough understanding of the technology.
Now make some notes that spell out what, if any, problems the engine might have been exhibiting while it was running. Did it start when cold easily? Did the car run rich (smell of gas)? Did it hesitate upon acceleration or have any "flat spots" while cruising? Were there any leaks? It's important to note such problems beforehand so you can look for them while the carburetor is disassembled. In the case of our subject Galaxie, the car stank of raw gas, ran very poorly, stumbled upon acceleration, leaked fuel onto the intake and had no choke assembly at all! It pumped out so much smoke from the exhaust that you could see it coming — or going — for miles. This thing desperately needed a rebuild. (In fact, the engine was ultimately contaminated with raw gas that, in turn, destroyed the bearings and rings.)
Holly carb, showing primary venturi (A), secondary venturi (B) and discharge tube (C).
As with any component that has lots and lots of parts, you need to approach this task with an organized procedure. So, let's start with a table, workbench, etc., with plenty of space to spread things out (3 square feet is nice) and lots of overhead light, because with any intricate task you need to see what you're doing. Next, spread the work surface with craft paper, brown wrapping paper or something else that will contrast with all your parts. Newspaper isn't a good idea, because little clips and e-rings will get lost in the print areas and will get thrown away. We speak from experience.
Open your rebuild kit box and empty the contents. Before opening the inner box of parts, unfold the instruction sheet to verify it's the right kit for your model of carburetor. If it isn't and you've opened the parts box, they won't take it back! Lay the parts box aside for now.
We recommend cleaning the exterior of carburetors after disassembly rather than before, because it's possible to push dirt inside. Next, follow the instruction sheet and start taking the external parts off the carburetor. Keep all parts in order as you disassemble.
Procedural note: Always, always lay out the parts in order of disassembly. The large area on your work surface will allow you to do so. If parts are scrupulously kept in order you'll never have to figure out how they go back together.
Following disassembly, soak all parts (except those made of rubber, leather, fabric, electrical components, diaphragms, plastic) in clean carburetor cleaner. Carburetor cleaner can be found in tubs and spray cans and is typically tri-clorethylene based. It's toxic to breathe, to make sure the area is well ventilated. An alternative to using carburetor cleaner is to use strong detergent (liquid dishwasher detergent works well) and hot water. Parts will have to soak for a while, but will come out clean. An advantage is that the other parts that are damaged in solvent can be cleaned this way. Clean parts one-at-a-time if you aren't familiar with order of assembly, or use a container that is large enough to keep everything in order.
When everything's clean, rinse off and blow dry. You'll need an air hose or one of those cans of air sold by electronic stores. Blow out all passageways and any hole — of any size — you can find. Never, never use wires or drill bits or dental picks to probe into orifices, as they are carefully engineered to a specific size and shape. Blow parts dry until no trace of liquid remains. Feel free to rinse parts in denatured alcohol.
Time to refer to the instruction sheet and lay out the new parts. The first thing you'll notice is that there are more gaskets and other parts than your carburetor could possibly use. That's okay, because the kit has parts for every different configuration in which your carburetor was manufactured. Take your time and lay out the correct parts you need to use, then put all the rest aside to avoid confusion. One by one, we will replace each used part laying on the work surface with its corresponding new one as we put the carburetor together. However, we need to do this slowly, checking adjustments along the way.
One of the most misunderstood, and therefore improperly adjusted, components in a carburetor is the float valve. They are simple devices that work very much like the float valve in a toilet, but are nowhere near as "forgiving" in their design. Each engine configuration requires its own float-setting: that is, a specific fuel level must be maintained during operation. If this fuel level is too low the engine will be fuel-starved under acceleration, in sharp turns and at full throttle. If the level is too high the engine will get flooded, run too rich or not respond to adjustments. Our Holley's float setting, according to instructions, required us to adjust the valve body until the float is parallel to the housing wall. We tested the float valve by blowing into the fuel fitting, making sure the valve closed off fully when inverted. Most rebuild kits, by the way, provide a measuring device to assist you in setting float levels. Use it!
Tip: If no measuring device is provided, use a drill bit as a precision measuring tool. Most drill bit selections come in 1/64th inch increments, so it's easy to find one that comes very close to the required float setting.
Every carburetor precisely meters the flow of fuel. To do so, very carefully machined orifices are used, called "jets." The jets may meter fuel on their own or be used in combination with moving tapered needles, but the outcome is the same — the engine gets the right amount of fuel at the right time. Holley carburetors use easily-replaceable jets that are available in a wide range of sizes. It's important, therefore, to consult the factory specifications to ensure that correct size jets are used (if the engine has been modified, the jet size must be changed accordingly). We did so for our rebuild project and found the jets in the carburetor were too large. New jets were obtained and installed.
The "heart" of the carburetor is the throttle plate. Its movement dictates how the engine runs, and if the plate is bent, dirty, worn or otherwise not fitting well the engine will experience poor idle, low-speed response and other problems. Our plates (two of them, one for each barrel) were filthy and sticking on the venturi wall at the closed position, so we took them off the throttle shaft, sliding the shaft out of the carburetor body.
Disassembled throttle Plate assembly.
We also wanted to see how much "slop" there was in the shaft's fit through the holes. Over time throttle shafts wear in their sleeves, gradually making the holes elliptical in shape. This creates an air space, in turn causing a vacuum leak. Our shafts and sleeves were fine, but if they were worn badly we would have had to replace the carburetor body or find some way of fixing the problem.
After cleaning and polishing the plates, we re-installed the shaft and positioned the plates carefully to ensure proper operation. We backed off the idle speed screw to allow the plates to contact the venturi wall simultaneously, allowing them to rest against the wall gently. We then tested the throttle operation to make sure there was no sticking or binding.
Most carburetors used on domestic cars utilized a power valve and accelerator pump. Contrary to popular belief, they are not one in the same, but perform different functions at different times (time to re-read the theory?). Our Holley's power valve is inside the float chamber, screwed into the central casting of the carburetor. It isn't adjustable, but our kit provides several to choose from. We only need to consult our instruction sheet to find the model # carburetor we're working on, then find the correct power valve for its application. The correct gasket was chosen and the power valve screwed in tightly.
Next, we assembled our accelerator pump. It is screwed to the base of the float chamber where there is a little hole for gas to fall in. The rubber diaphram (a new one, of course) is actuated by linkage operated by the throttle shaft, in turn "spritzing" a measured amount of fuel into the venturi. We tested it for smooth operation after assembly.
The middle block contains the jets, power valve and idle mixture screws. We double-checked all passageways were clean and unobstructed, then cleaned and lightly polished the points of the idle mixture screws. We then put them back in, installing new o-ring gaskets.
Okay so far, and it's getting time to start putting the big pieces together. We selected the proper gaskets that get sandwiched between the float chamber, middle block and carburetor body and held them in place carefully while inserting the four screws that hold everything together. These were tightened correctly. Then we attached the bottom plate to the carburetor, making sure the gasket was in place and facing the correct way.
All moving parts were again tested for smoothness and full travel. We gently screwed in the idle mixture needles, then backed them off one turn as a preliminary setting.
Once the carburetor (in this case, three) is bolted back on the intake and all lines, hoses, linkage, etc. are attached, it's almost time to tune up the system. Almost? That's right; almost, because the worst mistake anyone can make at this point is to set up the fuel system without first making sure the ignition system is in perfect adjustment.
Got that? If the engine's mechanical timing isn't matched perfectly with its electrical timing, adjusting the fuel system will simply start a dog-chasing-its-tail situation that can never be fixed. Go through the entire ignition system (point gap, dwell, timing advance at idle, total timing advance, plug wires, plugs, firing order) and make sure it's dead-on. To test timing advance you'll need to get the engine running, but don't make any carburetor adjustments.
Now that the engine should run correctly, get it started and warmed up. It may not idle without giving it some throttle, since the throttle plate (idle speed) screw was set at its bottom limit. Once the engine is running at proper temperature, increase the idle speed screw until it barely keeps running at idle. Then, start backing off one of the two mixture screws — slowly — until the engine speed starts to increase. Keep backing off until the speed no longer increases, but not more than 1 turn. Go to the other mixture screw and repeat the procedure.
At this point the engine is running smoothly. Adjust the idle speed (throttle shaft) screw until the desired idle speed is reached. Test the accelerator linkage, run the engine up in speed and look for leaks and loose connections.
Many believe there are only two ways to adjust multiple carburetors: almost and not at all. In fact, they are easy as long as you keep in mind that they all must work in synchronization. Therefore, all must be jetted equally, all must have the same idle mixture setting and all must be operated properly by the linkage. In the case of our Galaxie, all three carbs have idle mixture screws, but only one has a choke. They are set up "progressively," that is; the center carburetor is the "master" that gets the car off the line and moving, then as its throttle plates open about 1/3 of the way the linkage opens the throttle plates on the rear unit. As the rear unit's throttle opens to 1/3, its linkage starts opening the front carburetor's throttle plates. The linkage is designed so that, by the time the gas pedal is fully depressed, all three carburetors are fully open.
Logic dictates that three carburetors would be fitted with main jets smaller than those fitted in a single carburetor on the same engine, since only a finite amount of fuel can be used by the engine. This is true, but overall mileage will be much worse with three carburetors because there are three idle systems (the most fuel-intensive part of running an engine). Also, cars with three carburetors tend to be driven more aggressively. The more one puts his foot into it, the more fuel is gulped.