Carburetor Rebuilding: No Big Deal, Really - Page 2

The Carburetor Jets

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.

Throttle Plate

Throttle Plates

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.

Idle orifices.

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.

Carburetor Power Valve And Accelerator Pump

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.

Middle Block

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.

Now What?

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.

Tuning the Carburetor?

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.

Multiple Carburetors:

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.