Mark Houlahan
Tech Editor, Mustang Monthly
December 18, 2013

Edelbrock Carburetors

As the king of aftermarket intake manifold design and production, Edelbrock certainly knows a thing or two about airflow and an engine's fuel demands. Instead of designing a new carburetor from scratch Edelbrock engineers started with a known good product and simply made it better. If the Edelbrock Performer and Thunder series carburetors look familiar to you, then most likely you've messed with a pal's Carter AFB or AVS carburetor back in the muscle car heyday. Today, Edelbrock has refined the original Carter design and offers 16 Performer versions from 500 to 800 cfm with manual or electric choke, and available in satin, black powdercoat, EnduraShine, and vintage look. The Thunder series comes in 18 part numbers from 500 to 800 cfm in manual and electric choke, with satin and EnduraShine finishes.

Moving up to Edelbrock’s Thunder series, which utilizes the later Carter AVS design, nets you an upgrade to the adjustable air valve secondary plate (similar to the new Street Demon) that is adjustable on the carb with no extra parts required. The Thunder series also features secondary booster venturi clusters.
In this cut-a-way you can see the accelerator pump circuit of the Edelbrock carbs. Unlike a modular carb design that uses a square diaphragm and pivoting lever, the Edelbrock design uses a piston with a spring that is actuated via external linkage. This linkage can be adjusted (bent) or positioned in different actuation lever positions. Just be aware that the accelerator pump cavity shares the fuel level with the main body, so if the accelerator pump is raised higher than the fuel level, you’ll have trapped air to compress before you get your “pump shot.” As such, overall float level is critical to accelerator pump function.
Another way to improve the accelerator pump’s function is to change the discharge nozzle size. With a single twist of a screw you can change out the factory nozzle to one of the three in Edelbrock’s #1475 accelerator pump nozzle kit. If your Edelbrock has a stumble on acceleration, often the discharge nozzle size is usually the issue.
While the Demon uses a three-step metering rod design, the Edelbrock carburetors use the traditional two-step metering rod. The metering rods might look similar, but they should not be swapped between the different companies’ offerings.
Like all carburetor manufacturers, Edelbrock offers a slew of tuning, service, and rebuild parts. For the typical Edelbrock carburetor, the calibration kit for the model carb you’re tuning is the best place to start.

Holley Carburetors

Holley's four-barrel carburetors have been a staple of the aftermarket performance scene for half a century and have been used by Ford for OE carburetion solutions since 1957. It's no wonder that the company's modular design has been the basis for a whole cottage industry of upgrade parts and complete custom carburetors for motorsports solutions in everything from off-road buggies to oval track racing. Lift the hood of a classic car at a cruise night or test-n-tune at the track and your chances of finding a Holley four-barrel sitting on the intake manifold are better than average. Over the years, the Holley design has changed little, but constant improvements keep the Holley 4150/4160 line of carburetors at the forefront.

Moving to the 4160-based Holley, you’ll notice a few differences. For 4160 Holleys up to 600 cfm, the carburetor comes with side-hung floats and a single fuel feed with an external transfer tube for the rear fuel bowl on the driver side. You’ll also note that there is no visible metering block at the rear of the carburetor.
Moving to the 4160-based Holley, you’ll notice a few differences. For 4160 Holleys up to 600 cfm, the carburetor comes with side-hung floats and a single fuel feed with an external transfer tube for the rear fuel bowl on the driver side. You’ll also note that there is no visible metering block at the rear of the carburetor.
By comparison, this is a 4150-based Holley Double Pumper—a mechanical secondary carb—in Holley’s popular tumble polished finish. Note the lack of vacuum diaphragm housing. Also visible is a manual choke cable bracket and “four-corner idle circuit” with idle screws in the rear metering block.
When you use a 4160 Holley over 650 cfm, the carb will come with the more traditional looking center-hung fuel bowls and dual-feed fuel inlet on the passenger side of the carb.
While the 4150 Holley uses a secondary metering block as well (with or without the power valve), the 4160 model uses a metering plate. The plate is a cast plate with specific sized holes for fuel metering. While it is common for Joe Shadetree to drill these out, you can only go larger. The better bet is to order the properly sized metering plate from Holley. The rear fuel bowl completely covers the metering plate, and as such, needs to be removed for access to the metering plate for service/tuning as well.
All Holley modular carburetors use a metering block for the primary venturi side of the carburetor. These blocks hold the main jets and a power valve. Accessing the metering block for jet changes or to replace the power valve entails draining the fuel from the fuel bowl and removing the fuel bowl for access.
The Holley’s accelerator pump system differs from others in that instead of bending a linkage or changing the linkage mounting hole position, Holley uses plastic cams on the throttle arm. Holley’s accelerator pump cams come in a nine-piece kit with each cam offering two mounting positions. That’s 18 different accelerator pump profiles to choose from. Of course, the accelerator pump discharge nozzles are available in numerous diameters for further tuning of the accelerator pump shot as well.
Some of Holley’s newer four-barrel offerings, such as its Street Avenger aluminum four-barrel, include Holley’s quick-change vacuum secondary spring kit as standard equipment. If your Holley does not have this upgrade, you can purchase the kit separately. And it makes easy work of tuning the secondary throttle opening on vacuum secondary carburetors.
Holley offers replacement jets in numbered pairs, 10-packs, and this huge tuning assortment. Holley jet numbers range from 40 to 110, with inside diameter increasing as the jet number increases. Tuning the jet sizing can be accomplished with road/track testing or a dyno (engine or chassis).

Required Reading

As you might suspect, we're just touching the tip of the carburetor tuning iceberg here. The numerous carburetor part numbers, features, and options would take every page in this magazine to thoroughly discuss. As such, we can't recommend the proper educational material enough. Starting with tuning/rebuilding manuals, either from aftermarket publishers or from the carburetor companies themselves for an in-depth look at how carburetors work in general, as well as the specific functions of your model carb. You'll also find helpful exploded views/cutaways, FAQs, and tuning videos on each manufacturer's website as well. Many carburetors today come with an installation and tuning DVD right in the box—don't chuck the disc in your pile of paperwork on your workbench—pop it into your computer or DVD player and watch it.

Keep it Clean
A clean fuel system is paramount to proper function of your carburetor. From the moment the fuel is pumped into your vehicle, there are numerous chances for it to pick up dirt and debris that will foul your carburetors intricate fuel and air passageways. This can lead to a poor running, stumbling, stalling vehicle that will have you cursing the gods of carburetion. Keep these simple basic facts in mind and your carburetor (and fuel system) will provide you trouble free operation.

Always run an air cleaner
Street cars aren't race cars running for nine seconds at a time two or three times on a weekend! Having an air filter in place ensures the air entering the carburetor (and thusly the engine) is clean of dirt and contaminants that can foul a carburetor (and ruin the internals of an engine over the long term).

Always run a fuel filter
A quality fuel filter should be used between the fuel pump and carburetor. Do not use cheap plastic filters and if you have a glass filter for easy viewing, get rid of it now. Glass does not belong in a fuel system. A high-flow aluminum filter with replaceable or cleanable filter media is best.

Keep your air bleeds clean
Dirt and crud can find its way to your carburetor's air bleeds in the air/choke horn area. Keeping them clean ensures the carburetor gets the air it needs through the right passages for the carburetor to work. A simple blast with carb cleaner to clean away dirt is all you need. Do not run a wire tool through the air bleed to clean it; you will damage the air bleed.