Tech | Carb Tuning Secrets
For nearly 100 years, a carburetor has been at the heart of almost all internal combustion engines. Only in the last 20 years or so have car makers used electronic fuel injection exclusively on gasoline engines. There have been examples of mechanical fuel injection for automotive applications in the past, and of course diesel engines use fuel injection. However, the classic Ford cars we love were all carburetor-equipped, from the 170-cubic inch six-cylinder with a one-barrel Autolite carb to an R-code 427 with eight barrels of Holley induction in the form of dual-quads. Carburetors still compete effectively with fuel injection on engines when it comes to making power and when we do an EFI conversion, horsepower production is not the single largest motivation for doing so. The fact is that when the time comes to run the quarter-mile or make a few pulls on the rollers, a carburetor gets the job done just about as well as fuel injection. Even though simple in principal, in that a carburetor functions as an atomizer, there are many subtle variables to be considered and adjustments that can be made to get your engine running its best.
For this discussion we're...
For this discussion we're going to look at carburetors that have a central body and separate metering blocks and float bowls such as those from Barry Grant, Quick Fuel Technology, and Holley; and we'll save the Carter and other style units for another overview. This exploded view of a Barry Grant four-barrel shows the base plate (4), central body (3), two metering blocks (2), and float bowls (1).
Of course, once engine modifications begin then things become a little more involved. When one enthusiast decides on a mild 289 with single exhaust, while another equips his K-code with long-tube headers then carburetion requirements will differ. However, two rules of thumb apply across the board when it comes to carburetor tuning and selection. The first is sizing the carburetor appropriately to the displacement or flow capacity of the engine. For example a 750cfm carb would be too large a choice for an A-code 289 that the factory equipped with a 480cfm unit. However, adding compression, cylinder heads, and a higher-lift camshaft increases the flow capacity of the engine and then a larger carburetor might be called for. A 600cfm unit is a good bet for a healthy short-deck engine while a 650 to 750 cfm would be a good size for a stock or stroked Windsor engine. Save the 850 size for your 428 or larger displacements or otherwise radical engines. According to the experts at Pony Carburetors the following table lists approximate flow capacities of different Ford engines in their stock state. These numbers are calculated at 90 percent volumetric efficiency.
| Displacement | CFM Needs |
| 289/302ci | 453 cfm |
| 351ci | 550 cfm |
| 390ci | 609 cfm |
| 428ci | 668 cfm |
This illustration is also...
This illustration is also courtesy of Barry Grant and it shows how the fuel (in red) is transformed from a liquid state to an atomized state as it passes through the carburetor, picking up air form the air bleed passageways.
When we spoke to various experts in the carburetor field about the most common problems that folks encounter with a carburetor on a street performance car they cited stumble off idle, bogging at wide open throttle application, secondary engagement issues, and lean or rich operating conditions. These problems can be addressed with adjustments to the float bowl fuel level, accelerator pump, secondary opening systems, and jet settings. In this short overview let's examine these common engine performance complaints that can be addressed by simple carburetor adjustments and then look at a dyno-documented example of the benefits achieved when carburetor size and engine timing are optimized for the engine.
Matching the carburetor size to the capacity of the engine ensures good throttle response throughout the rpm range and also the best economy of operation.
This diagram illustrates Bernoulli's...
This diagram illustrates Bernoulli's principle and shows how fuel is drawn into the slipstream by the low pressure created because of the diameter restriction at the center of the venturi.
The second rule of thumb that the experts we spoke to stressed over and over again is that a properly tuned carburetor will be complemented most in terms of performance by the correct engine timing. Correct initial timing for most stock V-8 engines occurs at 10 to 14 degrees before TDC. On engines with high-
performance components added, the initial ignition timing should be advanced. For example, high-performance camshafts often have a longer duration for valve events and so they generate less depression or signal at the carburetor at idle. As a result air-fuel droplets are bigger, atomization is poorer, the burn rate is slower, and the combustion more incomplete. To overcome these tendencies initial timing should be increased.
Use a filter to avoid carburetion...
Use a filter to avoid carburetion problems to begin with. An inline unit like this one from Barry Grant is a good way to stay out of trouble.
Before anything else, be certain that a clean and filtered supply of fuel is provided. Contaminated or dirty fuel is one of the largest causes of improper carburetion. When installing a carburetor, renew the fuel filters. Place one directly before the fuel pump and the other before the carburetor.
Float Bowl Facts
After a rebuild or before attempting any other adjustments, checking and adjusting the float level comes first. This should be the first thing you do before attempting to make any carburetor adjustments. The float functions like the float in a toilet tank and shuts off the flow of incoming fuel entering the bowl by closing a needle on the seat of the fuel inlet. The float level should put the fuel level just below the bottom of the sight plug hole if your carburetor is equipped with one. You can make the adjustment with the vehicle on a level surface and the engine idling.
The BG inline filter uses...
The BG inline filter uses a replaceable cartage filter for a first class filtering job. Most new carburetors come adjusted to run out of the box and the way we get into trouble is by overlooking the obvious things like clean fuel.
This cut away diagram shows...
This cut away diagram shows how the float mechanism works. When the fuel level in the bowl rises to the correct level, the float closes off the needle against the seat. Incorrect float level can be at the root of several different problems, including poor idle and engine flooding.
First, remove the sight plug....
First, remove the sight plug. We had a shop towel at the ready for any fuel spillage. If your carb is equipped with a clear sight plug you won't need to do this and will be able to sight the fuel inside the bowl with a flashlight.
To make your adjustment you...
To make your adjustment you will need to loosen the lock nut on the needle and seat. This will allow you to turn the adjusting nut to raise or lower the float level. Each hex flat on the nut will change the float level approximately 1/32-inch. When you have the fuel level just below the sight plug hole you will then tighten the lock screw and reinstall the sight plug. Make sure you have a shop towel handy in case you have any fuel leaks from the sight plug or needle and seat adjusting nut.
If you'd rather perform an...
If you'd rather perform an exact measurement or your carb isn't equipped with a sight glass or hole you can remove the float bowl from the carb as shown. If you carb is on the engine you'll need to drain the bowl first.
Once the bowl has been removed,...
Once the bowl has been removed, the float can be set to factory specifications using any precise measuring device. Repeat the procedure for both primary and secondary float bowls. With the bowl held upside down, the distance to the bowl ceiling is measured and set at the desired distance.
This photo shows what your...
This photo shows what your accelerator pump shot should look like. If you don't see fuel discharged into the carb like this, then look for problems with the accelerator pump. The fuel is shown squirting out of the accelerator pump nozzles, which can be changed in size to control fuel delivery speed.
The most common cause of a stumble off-idle or bogging is an inadequate or incorrect accelerator pump shot. The first thing to do is to look at the discharge nozzles and make sure you are getting a good, strong shot of fuel when you pull the accelerator linkage back.
If there is no shot of fuel then you need to inspect the pump diaphragm for a hole or tear. You will also need to make sure that the pump passage is clear from any trash or debris. The accelerator pump system consists of three main components: the pump diaphragm, the pump cam, and the pump nozzle. This is the system on your carburetor that is most responsible for having good, crisp, off-idle throttle response. Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened. By doing this, it acts to smooth the transition between the idle and the main operating circuits so that no stumble, hesitation, or sluggishness will be evident during this transition phase.
Very often the source of the...
Very often the source of the problem is a perforated or otherwise damaged accelerator pump diaphragm. Replace the diaphragm with a new unit. This collection of accelerator pump parts shows the diaphragm in the lower right corner of the photo.
Once this clearance has been set, make a careful inspection of the pump linkage and work the throttle. Make sure that the accelerator pump arm is being activated the moment that the throttle begins to move. This will assure that pump response will be instantaneous to the movement of the throttle. These adjustments can be made by turning the accelerator pump adjusting screw that is located on the accelerator pump arm together with the pump override spring and lock nut. The amount of fuel that can be delivered by one accelerator pump stroke is determined by the pump's capacity and the profile of the pump cam. The period of time that it will take for a pre-determined amount of fuel to be delivered is affected by the pump nozzle size. A larger pump nozzle will allow fuel to be delivered sooner than a smaller pump nozzle. During acceleration tests, if you notice that the car hesitates at first and then picks up, it's a sign that the pump nozzle size should be increased. A backfire or lean condition upon acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size may be called for.
Throttle at wide open application often calls attention to secondary opening issues. To see if your vacuum-operated secondary is opening, its operation can be checked using a paper clip. The engine needs to be under a load before they will open. Take a normal paperclip and clip it onto the secondary diaphragm rod. Then push it up against the bottom of the secondary diaphragm housing and go out and drive the vehicle including a pass at WOT. When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod, then you can tell that the secondary has opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.
With a good diaphragm in place...
With a good diaphragm in place the first adjustment to check is the clearance between the pump operating lever and the pump diaphragm cover's arm, at wide open throttle. This clearance should be around 0.015-inch. The purpose for this clearance is to assure that the pump diaphragm is not stretched past its maximum limit at wide open throttle, which would cause premature pump failure.
Secondary Questions
This...
Secondary Questions
This Holley 4160 carb is a popular choice for a mild performance car with an automatic transmission. It has a single feed fuel line with no secondary metering block and a vacuum operated secondary. The black secondary actuator is seen on the side of the carburetor body and it contains not only a vacuum diaphragm but a calibrated spring that controls secondary engagement speed.
To change secondary opening...
To change secondary opening rates, different calibration springs can be interchanged. Variables in secondary spring selection will include vehicle weight and transmission type. For street cars, the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a mechanical secondary (called a double pumper) because they work by sensing engine load. This mechanical secondary carburetor is best on a lighter car with a more radical camshaft. In addition to the mechanical secondary actuation the rear bowl also has another accelerator pump to compensate for the immediate inrush of air that results when the secondary snaps open. This is what is meant by the term "double pumper." A lower axle gear ratio and manual transmission work best with a carb of this type.
To access the jets, the float...
To access the jets, the float bowls need to be removed. Many people simply remove the metering block(s) of the carburetor altogether since power valve changes and new gaskets are often installed at the same time.
Jet size changes can be called for if the engine is determined to be running too rich or too lean. An air/fuel meter used during a dyno session concluded that our engine was running in a too lean condition at over 14:1 air/fuel ratio at wide open throttle. We knew that besides being a dangerous condition, more power could result with a jet change on the secondary side of the carburetor.
Back on the bench we had a...
Back on the bench we had a wide selection of jet sizes. Our car was running 73s for the primary and secondary. We went to a 76 and then a 79 for the secondary, trying both and gaining a little each time.
We use a special tool that...
We use a special tool that allows us to change jets quickly and easily, but a proper sized screwdriver will do the trick if you're careful. When we finally settled on an 82 jet, our mixture was much better at 12.8 to 13:1 air fuel ratio. We also picked up 11 horsepower and 12 lb-ft of torque in the process.
While other type of carburetors...
While other type of carburetors use metering rods to add additional fuel under moderate to heavy loads, this type of carburetor uses a power valve. Power valves can also be changed to adjust how much and when the additional fuel is added. Power valves used to be susceptible to blow out because of backfire but now a check valve prevents this problem in modern carburetors. Consult manufacturer recommendations when making power valve selections.
Here's the car's single-feed...
Here's the car's single-feed 600cfm carburetor in place. While the car ran fine, we suspected a little bit more airflow wouldn't hurt this engine with its top-notch AFR cylinder heads and lumpy Comp roller cam.
Once you've got your engine running right with any carburetor issues ironed out, is it possible to wring more performance out of your engine by going to a larger carb size? We wanted to find out so we took our car, which was running fine with a 600cfm Holley four-barrel on it and decided to step the carburetor size up a bit. The 351W engine has AFR aluminum cylinder heads on it, headers, and a reasonably rowdy roller cam. We felt fairly confident that the engine had enough oomph to absorb and successfully utilize some extra induction capacity. We wanted to see what differences might emerge when comparing a Holley 4160 600cfm single-feed carburetor with vacuum secondaries to a 4150 650cfm double-pumper with a mechanical secondary.
The new 650 drops right into...
The new 650 drops right into place over the Edelbrock Performer 351W dual-plane intake. Most everything attaches in the same fashion as the 600. The flange nuts should be tightened in an "X" pattern so as not to damage the carb's throttle body. The only change that really needed to be made was to make up a new fuel line for the 650's dual-feed fuel inlet system.
After testing the new carburetor...
After testing the new carburetor at the same timing setting, we bumped the timing up on the engine to get it dialed in with the new carburetor. We advanced the timing to 14 degrees initial and 34 total and put in some 91-octane gas. Overall, the new carb and increased timing made our test session a success as we picked up well over 20 horsepower at the tire. The combination of the swap and setting the timing at 34 degrees total advance got us right up to the 320-rear-wheel-horsepower mark with 320.2 at 5,400 rpm-an improvement of 15.9 horsepower on top of the carburetor swap.
We visited Superior Automotive...
We visited Superior Automotive in Anaheim, California, for baselining our car with the 600cfm Holley. Our result was a respectable 298.7 horsepower at the tire at 5,200 rpm. Timing was set at 10 degrees initial with 30 degrees all in at 3,200 rpm.
Here are the dyno results with the new carb and the same timing. A simple carburetor swap to the slightly larger 650 got us to well over 300 horsepower with a solid 304.3 horsepower at the tire at 5,300 rpm. Although the difference in peak-to-peak horsepower is only 5.6 hp, the curve is better overall. For example, at 4,000 rpm the difference is 7.6 more horsepower with the 650cfm carb. The bigger carb made a nice improvement on torque. The peak improvement in torque output ended up being 9.1 lb-ft, or 344.2 lb-ft with the 650 carb as opposed to the 335.1 lb-ft reading we got with the 600cfm carb.
| Baseline | Carb | Timing Change |
| RPM | HP | TQ | HP | TQ | HP | TQ |
| 3,000 | 182.0 | 318.6 | 175.1 | 306.6 | 186.4 | 326.3 |
| 3,200 | 196.0 | 321.7 | 202.0 | 331.5 | 210.4 | 345.4 |
| 3,400 | 207.3 | 320.2 | 219.5 | 339.0 | 227.6 | 351.6 |
| 3,600 | 226.2 | 330.1 | 235.9 | 344.2 | 240.1 | 350.3 |
| 3,800 | 242.5 | 335.1 | 247.3 | 341.8 | 247.9 | 352.0 |
| 4,000 | 250.7 | 329.1 | 258.3 | 339.2 | 253.1 | 349.9 |
| 4,200 | 261.1 | 326.5 | 266.6 | 333.4 | 263.1 | 345.4 |
| 4,400 | 270.9 | 323.3 | 273.5 | 326.4 | 273.9 | 342.5 |
| 4,600 | 277.1 | 316.3 | 277.8 | 317.2 | 282.3 | 337.0 |
| 4,800 | 284.7 | 311.6 | 282.1 | 308.7 | 287.8 | 328.6 |
| 5,000 | 294.4 | 309.2 | 293.3 | 308.0 | 296.9 | 324.8 |
| 5,200 | 298.7 | 301.7 | 302.4 | 305.5 | 309.1 | 324.7 |
| 5,300 | | | 304.3 | 301.5 | 314.2 | 317.4 |
| 5,400 | | | 302.7 | 294.4 | 320.2 | 310.7 |