• 2016

CARBURETOR TUNING

The 1960s and 70s was a time when automotive performance and big, powerful engines became synonymous with racing, restoration, car shows, auctions and cruises. Most muscle car engines from this period used carburetors as the basis for the induction system. By the early 1990s carburetors were replaced in production by multi-point fuel injection (MPI). MPI is expensive. Carburetors are still very popular today, in all the non-production applications. The basic 4-barrel carb was installed on small blocks (318 & 340) and on big blocks (383 & 440) so there is a wide range of engines to use as baselines. But, today’s performance approach yields a problem – more cubic inches, bigger cams, better heads, better exhaust all up the fuel demand which in turn, creates fuel tuning issues.


HOLLEY 4-BBL

You could call the Holley 750-cfm carburetor the standard in the performance industry. This specific version has vacuum secondaries, which allow it to work in street applications (and with automatic transmissions in performance competitions). While it is generally considered too large for 318 engines and too small for the big block stroker motors, it is the most readily-available vacuum secondary carb in the Holley line-up and therefore works well on all V8 engines. This 750 carb was not installed on Chrysler production V8s. However if you plan to replace the production 2- or 4-barrel carb, this is one of the most popular choices.

Jets for the Holley carb are readily available from Holley at most speed shops. With the vacuum-secondaries, diaphragm springs are used to control opening speeds. These color-coded springs are also available from speed shops (along with various sizes of accelerator pump nozzles). All of these pieces are used to tune the basic Holley carburetor.

BASIC TUNING

To tune any carburetor, break the operating range into 4 basic areas – idle, WOT (wide-open-throttle), rpm change, and part throttle or mid-range (driveability). Different carburetor systems use different parts to adjust or tune these areas. The idle is basically controlled by the idle adjusting screws (2) in the sides of the metering block and the primary jets. WOT is controlled by the primary and secondary jets. RPM change is controlled by the accelerator pump size and (when it opens while part-throttle/driveability) is adjusted by secondary timing (the diaphragm spring) and a sum of the other steps.

AVS 4-BBL

The AVS 4-barrel carburetor was designed and built by Carter and now Edelbrock. The AVS was installed on many Mopar® V8s in the late 1960s and early 70s. It replaced the older AFB carburetor. All service parts for the AVS are available from Edelbrock along with new AVS carb assemblies. The tuning steps are similar to the Holley except for metering rods and the adjustable air door. For tuning details, see HP Book #1555 on Carter/Edelbrock carbs.

THE 8-BBL

There were several 8-barrel carburetor systems used on production engines in either an in-line or cross-ram configuration. The last production version was built in 1971. Mopar Performance, Edelbrock and Indy Heads still offer 8-barrel intakes for various engines.

HOLLEY 3-2BBL or 6-BBL

One of the most unique carburetor systems is the Holley 6-barrel which is actually 3 Holley 2-barrel carbs mounted in-line. The basic system starts with a center carb like P4349237 and then uses two end carbs like P4349238 – one in front and one behind the center carb. Spacing of the carbs is the same on the big block and the small block. On the fuel side, they are connected by a unique three-Y fuel line. On the throttle linkage side they are connected by a very unique linkage (P4529061) which allows the center carb to open without opening the end carbs until the engine wants the extra air flow. The end carbs use a diaphragm (P4349321) like the vacuum-secondary Holley 4-bbl above and the color-coded springs are available from Holley. The three carbs bolt to the manifold using gaskets (P4529060) and the carb’s fuel bowl and metering block gaskets are P4349304. The two end carbs use a metering plate instead of jets for fuel regulation and these plates are available from Holley. Be sure to check model year, there are two styles available.

6-BARREL TUNING

The 6-barrel set-up is a little unique in its approach to tuning. For tuning purposes, look at the 6-barrel set-up as a 4-barrel with four secondaries, i.e. one carb not three! Buying used 6-barrel carbs is NOT recommended because it is almost impossible to tell what modifications have been done to them and if they have been modified, they should be sent to a professional rebuilder, which could be expensive. Even new carbs should be disassembled and checked out for the basics.

P4349241_Carburetor_0001First step in tuning – select the end carbs, and find the idle adjusting screws (2 – one for each throttle bore) which are in the front face of the carb’s attaching flange (base-plate). These are very small and delicate screws – be very gentle! Turn them in to seat them and then turn them out 1.5 turns on your flat-blade screwdriver. Repeat procedure for the second throttle bore, and then the other end carb.

Note: The center carb has the idle screws on the sides of the metering block.

Install the carbs onto the intake manifold and install the throttle linkage. Install the slide/pivot into the center carb and then install the rods to the end carbs. The end carb’s throttle should be fully closed during this process and each rod should slip easily into the hole in the end carb’s linkage arm. Once completed the two arms to the end carbs should form a straight line and should be basically horizontal. The center carb jets are two jet sizes smaller for the small blocks (up to 375 inches) – #62 jet, and #64 jet for the 440 big blocks – yellow springs for the end carbs in both cases.

The main metering restrictions, in the end carb metering plates, are .052″ (left and right) in both cases. As the engine gets larger and makes more horsepower, this restriction must get larger – .080″ to .090″ to .100″. These holes must be drilled out so be sure to have spare metering plates (can’t go smaller once it’s too large). The latest info in the 6-barrel tuning was done in HP Book #1528 written in 2008.

In most non-racing applications, you should use an air cleaner (non-cold-air). If you tune the carbs with the air cleaner off, then installing the air cleaner will enrich the engine. The 440-6bbl used a very special fuel filter (officially called a vapor separator). It should be replaced occasionally. If you plan to cruise with a small block, consider adding this fuel filter. The typical carburetor system uses a mechanical fuel pump like P4007039AB (big block). Today with these older muscle cars, they tend to be stored most of the time – consider adding an electric fuel pump at the rear, like a small Carter pump (local speed shop).

There are many more options for carburetors, jets and carb-tuning hardware. Tuning a specific engine package can be very time-consuming but, there are several books that can help.

byLarryShepard

TAGS:   |   |   |   |   |