I have an Edelbrock 2×4 setup using the 500 cfm Endurashine carburetors for my 383ci small-block Chevy that I’m putting in a ’53 Chevy hot rod pickup. In a month or so I’m going to fire the new engine and I’m wondering if there are any changes I should make to the carburetors before I take this out for its first test drive? I heard that I should lean out these carbs because they are a 2×4 setup. Thanks.
Jeff Smith: I’ve never had the chance to play with Edelbrock’s 2×4 setup, so I did a little research. The combination that you describe appears to be EDL-20254. As you mentioned, this kit is based on a pair of 500 cfm Edelbrock carburetors and comes with all the other necessary parts including the intake manifold, gaskets, linkage, fuel line and hardware. The primary carburetor has an electric choke while the secondary carburetor does not.
Since I don’t have any personal experience with this kit, I called my buddy Smitty Smith at Edelbrock to learn a little more. He actually has this exact kit on his 383 cubic-inch small-block Chevy-powered ’37 Ford pickup. In the course of our discussion, he mentioned that the system is designed so that the primary carburetor is mounted at the rear, rather than the front. He also mentioned Edelbrock did some dyno testing and determined that with this pair of carburetors on a typical street small-block Chevy, the jetting needed to be leaned out.
The stock jetting on a single-carb application would use an 86 primary jet with a 65×52 metering rod matched with 95 secondary jets. Let’s take a minute to explain how this metering rod system works.
The Edelbrock carbs are designed to use the combination of a metering rod and jet on the primary side and just a main jet on the secondary side. This offers more precise primary fuel metering for part-throttle operation.
The tapered metering rod is designed to fit inside the main primary jet with its vertical location controlled by the power piston. This piston is pulled down by engine vacuum and raised by a small power valve spring underneath the piston. Under light throttle, for example, the engine creates high manifold vacuum since the throttle plates are nearly closed. The vacuum pulls down on the piston, against the spring load. This also lowers the metering rod into the jet. Since the metering rod is tapered, the thick part reduces the jet’s flow area, reducing fuel flow in the main metering circuit.
When you stab wide-open throttle (WOT), manifold vacuum falls off to near zero. The spring then pushes the metering rod up and out of the jet, exposing the thinner, tapered end of the rod in the jet which allows more fuel into the main metering circuit. This becomes the power enrichment circuit.
For the 2×4 package, everything works exactly the same way but the jets and metering rods are different. Edelbrock has leaned out the primary circuit by adding a 65×57 metering rod that is larger on the small tapered end to flow less fuel compared to the single four-barrel 65×52 primary rod. On the secondary side, Edelbrock has reduced the secondary jetting from a 95 to a 77 because there are two carburetors flowing fuel.
Smitty says his engine runs great with the jetting right out of the box, but he warns that some enthusiasts don’t realize how critical ignition timing is to both performance and drivability. A typical small-block like the engine you have may well be equipped with aftermarket heads and camshaft. Unfortunately, many enthusiasts don’t add more initial timing with these combinations — relying instead on placing the initial timing at six or eight degrees before top dead center (BTDC) as called for stock.
Most small-blocks fitted with heads and a camshaft should start with much more initial timing — 14 to 16 degrees of initial timing is a great place to start. Then you need to measure the total timing advance above 3,000 rpm. As an example, let’s say you place the initial at 16 degrees BTDC and then discover that the distributor offers total mechanical advance of 32 degrees BTDC at 3,200 rpm. Generally, 34 to 36 BTDC is a better starting point. The simple fix is to just add another two degrees of timing to the initial and you will be close.
If you measure in excess of 36 degrees BTDC, then the best procedure is to modify the mechanical advance curve in the distributor so that it delivers somewhere near 36 degrees of total timing with the 14-16 degrees of initial. If you have an MSD distributor, the advance curve can be modified easily by changing the bushing in the advance weight area. We’ll save that specific discussion for another time, but the point here is that proper initial and total timing advance is important to get the most out of the engine once you have all the parts in place.
With the proper timing, the engine should run well.
If you think you might want to perform further tuning changes to improve the part-throttle performance, this is where the Edelbrock carburetors are really much more tuner friendly than other carburetors. Remember the primary metering rods mentioned earlier? These rods are easily accessed by loosening a small screw that allows you to slide the cover aside. There are two covers, one for each primary venturi. With the cover moved aside, the metering rod and piston will be pushed up by the power valve spring.
This makes it easy to change the metering rod without having to disassemble the carb or remove the lid. You will need to pull the top of the carb if you want to get at the primary or secondary jets. We mentioned that the stock 2×4 primary rod measured 65×57. The larger 65 number is the large diameter of the rod that sits inside the primary jet during high manifold vacuum cruising. If you decide to lean the cruise circuit at part throttle, you could change the two primary rods on both carburetors to a 67×55 combination. This will reduce the fuel flow at light throttle while adding a little more fuel under high power demands.
As you can see, there are many things you can do to fine-tune your particular system. Most enthusiasts don’t bother with all this because the engine will probably run fine right out of the box. But since all combinations and engines operate differently, don’t be afraid to make small changes to see if the engine responds. We’d also recommend keeping record of your changes just in case you want to go back to a certain setting that performed well. Have fun!