Q&A / Tech

Ask Away! with Jeff Smith: Tuning Tips for Edelbrock 2×4 Carb Setup on a Small-Block Chevy

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.

T.M.

Edelbrock 2x4 setup

(Image/edelbrock.com)

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!

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13 Comments

  1. Jeff – Read your reply to the guy that has the Edelbrock 2X4 carb setup. Back in the late 50’s – early ’60’s we ran a ’57 Chev. with the stock 270 — dual quad setup that came from the factory. Still have the complete carb system, manifold, distributor, etc. that came with the car. Any carb adjusting hints? Would like to use it on my ’77 Chev. Suburban with a small block 400 engine. Will it fit? Any help or suggestios would be appreciated.

  2. Pingback: 68 Running Rich? - Pontiac GTO Forum

  3. Jeff Smith says:

    While this may not be what you want to hear, a Performer RPM dual plane and a 750 cfm carb will be a lot better than that early ’60s 2×4 package. More torque and better throttle response -especially for a heavy Suburban that needs torque. But if you just want it to look cool with the 2×4 – I understand. One key would be to ensure the throttle shafts are tight to the base plates and those old carbs don’t leak into the manifold. I’m not that familiar with those older carbs. the manifold should fit as long as you are running the traditional small-block heads and not the Vortec or the later model small-block heads that use those weird vertical center four bolt holes. If you have either of these manifolds – that old 2×4 intake won’t bolt up. A big help when tuning the system is to balance all four idle mixture screws. You’d be surprised at how often I see mixture screws unbalanced – even when there are only two screws on one carburetor. Hope this helps…

  4. Hello small block 289 motor going to install dual 4;s this fall do the carb numbers have to match one has a factory chock other hasen’t one. Have been told both need to be non elec choke` so you can add your on one carb the rear matching carb’s I have two new carbs with less than 1000 miles on the run time one has a factory installed chock other doesn’t but the numbers don’t match has these on a big block chev and just seem to load up after eng was warm. Thanks for any Info you can send along Chuck

  5. Gerald W Hendon says:

    Hi Jeff have a blue print 355 stroker motor bp35512ct1 that I want to put the edelbrock dual quad carb with the 500 cfm carbs .What should i set the jets at so there is not too much fuel for that engine or are they right with the settings out of the box.

    • Gerald,
      I talked with Edelbrock about this and they have already made adjustments to the twin 500 cfm carburetors to lean them out. The only place where this might be an issue is on the idle circuit. But likely this will still be workable. If the adjustments are less than one full turn out, this indicates that the entire circuit is too rich, but install them first and then make careful adjustments. With four idle screws, a very small adjustment will have a greater effect. Hope this helps.

  6. I read on the dual quad that the font carb the idle screws should be half a turn out.The rear carb the primary should be tuned as per the instructions. What are your finding? Not to much documentation on the dual quad tuning

  7. wally hostetter says:

    i have a edelbrock duel quad air gap set up on my 1980 350 small block when i reach 6200 rpm the engine bladders ,be cause of two much fuel can you tell me what jets i need to install and metering needles to hopefully eliminate this problem,i was told that the cards they sold me were not jetted for a dual quad setup. i have tryed numerous jetting but can not seem to get just right.

  8. wally hostetter says:

    jeff i sent a reply, you answered me back with email about a bunch of questions that i answered to that email, have not heard back from you ???

  9. Jeff: I have a 327 small block in a t-bucket with an edelbrock duel quod set up. It is an older build ,but great motor. I want to replace the old carbs with 500 edelbrocks. SHOULD I put a progressive throttle linkage with the rear carb as the main one? Will they work well out of the box? It now has them both tied straight together an am having trouble adjusting them. I would like your input and thank you . From MIKE in N.C.

  10. Bruce Wilson says:

    Ok Jeff I have a 383cu chevy with the 2 X 4 and I see your recommendation for jetting which is a good start but I am at 6,000 ft should I down another 10%

  11. Bruce,
    A 10 percent number might be a bit too much. Holley suggests one jet size for every 2,000 feet of altitude. Each jet is roughly 2 percent flow change so for 6,000 feet – that would be roughly 3 jet sizes that would equal a 6 percent change. Jetting is also dependent on temperature, and humidity as well as pressure (altitude) so start with a roughly 6 percent change and see how that works and then fine tune from there.

    This is a really good question! Thanks

    Jeff

  12. Tom Alesi says:

    Hi Jeff,
    With this set-up, is it better to run a progressive linkage or a straight linkage? What is Smitty Smith linkage like? We are going to run 2- 500 cfm Edelbrocks on a 383 Chevy motor just like Smitty.

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