An excellent reference for setting up your Carter or Edelbrock AFB-style carburetor is Dave Emanuel's "Carter Carburetors" published by SA Design Books. However, for specific help in making these carburetors work well with in-line engines like Chrysler's slant six, I have had to do some research. While I was waiting for the parts to arrive so I could try using a QuadraJet on my Offy intake, I was also trying to get my AFB to run a bit better.

The problem I had been having was a bog off the line. I believe the problem was due to my lack of intake manifold heat because I was running headers previously and am now running Dutra Duals. When the throttle was quickly cracked open at low RPMs, the low vacuum and air flow caused the fuel to drop out of the air, thereby creating a lean condition. After trying the a number of metering rod and jet combinations from my Carter Strip Kit, I found that the metering rods with the largest difference between the economy step and the power step seem to work best. Since Federal Mogul is no longer making AFBs and Edelbrock has started making a new and improved AFB with the name Performer, I checked out their metering rods. Edelbrock has some metering rods with even larger differences between their economy and power steps so I tried the next size-difference larger. The performance of my car improved once again and the bog largely disappeared.

To completely eliminate remnants of the bog, there is a still a larger-difference rod available that I could have tried. I wanted to avoid increasing jet sizes because that would affect my cruising fuel economy and I think there is room to make the cruising fuel mixture a bit leaner.

After trying the Quadrajet manifold with the stock Chev 350 jetting and installing an intake manifold heater hot spot, I found that I had absolutely no bog on acceleration. If you are running a street engine, make sure your intake manifold heat control system is working properly before you try enriching your fuel mixture.

Another thing to try if you have the hood clearance is a carburetor spacer. Some people have found that they could eliminate their acceleration bog by raising their carb with the spacers that have separate holes for each barrel. The extra distance between the throttle plates and the intake manifold allows fuel to better mix with the air before it reaches the intake plenum. A carb spacer could also help to reduce percolation by better insulating the carburetor from the intake manifold. Keep hood clearance in mind as you experiment with carb spacers.


If you are using an Edelbrock or Carter AFB and are experiencing a bog at low RPMs, try experimenting with the following metering rods:

PN Cruise/Power Size Size Difference
16-241 (Carter) 0.0705 x 0.047 0.0230
16-690 (Carter) 0.0745 x 0.047 0.0275
#1459 0.075 x 0.047 0.0280
#1450 0.070 x 0.042 0.0280
#1455 0.073 x 0.042 0.0310
#1419 0.075 x 0.042 0.0330
#1449 0.070 x 0.037 0.0330
#1454 0.073 x 0.037 0.0360
#1458 0.075x 0.037 0.0380

To maintain your cruising fuel economy, make your rod changes with Edelbrock rods of the same size economy step. You can maintain a fuel mixture for a given rod step (economy or power) and change the fuel mixture of the other step by maintaining a constant flow area between the metering step and the jet.

 

The following links are to tables that relate the flow areas of the various combinations of metering rods and jets.

 

Let's use Edelbrock Performer Metering Rods and Jets for this example. If you are getting good fuel economy with MR#1459 and you are using Jet#1429, your cruising flow area is 359.4 square inches (to the –5 power). Jet#1429 and MR#1459 yield a power flow area of 627.7. If you find that your engine is a bit lean for full power, you can easily increase your flow area by 5.6% to 662.6 with no change in economy flow area by keeping Jet#1429 and changing to MR#1459.

However, if you find that your full power fuel mixture is a bit rich with MR#1459 and Jet#1429, you need to find a metering rod and jet combination that has about the same economy flow area but has a slightly smaller power flow area. In this case, you can try using MR#1453 with Jet#1427 which has a resulting cruising flow area of 358.4. Jet#1429 and MR#1459 yield a power flow area of 627.7. Using Jet#1428 with MR#1455, gives a power flow area of 580.8. In this case, you decreased your economy flow area by only 0.3% but you more significantly reduced your power flow area by 7.5%.

The only practical difference between the two tables is in the part numbers. The Carter table lists parts that were available in the now obsolete Carter Strip Kit. The Edelbrock Performer table lists parts that are currently available through Edelbrock. The metering rods, jets, and springs will interchange between carburetors, provided that the carbs using only the two-step rods are considered. The three-step rod carburetor is not offered by Edelbrock so don't try using their parts in one. Be sure of the type of carb you have before you buy the parts for it!

 


Set-Up Procedure

For those people who don't know where to start when they first install their carburetor, let me offer you the procedure I use. Please also to refer to page 32 of FM's AFB procedure while you are doing this. While this procedure was used to set up a carburetor on an inline engines, it should also be reasonably applicable to V-8s. Before you do any work, I would recommend that you track down Carter Carburetors by Dave Emanuel and study it well. Before you start, make a note of what metering rods and jets are in your carb.

If you have any intention to install headers, do it before you start setting up your carburetor because the availability of intake heat will affect the set up. I recommend that you keep the exhaust manifolds for street engines because you can retain the heat riser system. Personally, I think engines look so much cleaner with manifolds anyway. Without the heat provided to the hot spot in the intake manifold below the carb, the fuel mixture will have a tendency to lean out on acceleration because the fuel tends to fall out of the air to puddle on the floor of the intake. Having said that, if your V-8 intake manifold doesn't have an exhaust cross-over passage, you may want to consider removing the heat riser valve to slightly improve exhaust flow. Do not remove the manifold heat control valve on inline engines.

Before you do any metering rod and jetting changes, and assuming your carburetor is in good working order, install everything and see how well it runs. Once you have a baseline fuel economy and even a few runs down the drag strip, you will have some idea whether any changes are really improvements. Start by trying to tune the main metering system first. You could do this by either tackling the cruise mode or the power mode of the carb. If this is a daily driver for you, you might want to the cruise mode first. Otherwise, try setting up the power mode first. Start off with the weakest metering rod springs and the smallest accelerator pump shot.

For the cruise mode, the quickest and most reliable way of determining the best rod and jet combination is to drive at steady highway speeds long enough to get a repeatable gas mileage reading. Keep leaning it out until you get the best gas mileage or until you find the car has a lean surge. Your best gas mileage will likely be with the combination that is just richer than what causes a lean surge.

Remember, lean fuel mixtures burn slower. At some point, should also consider increasing your vacuum advance (with an adjustable vacuum advance) in 1-turn increments until you find the point where your fuel economy starts to drop off or begin to notice audible knock (or pinging). Then decrease your vacuum advance 1/4 turn at a time until the knock disappears or your gas mileage peaks.

The power mode is a bit trickier because too much acceleration will cause the secondary metering system to kick in. I would disconnect the secondary linkage so that your carb functions as a 2bbl. Be careful not to lose the clips. When accelerating, too lean a setting will cause an off-idle bog when the fuel falls out the air stream as manifold vacuum drops. A somewhat scientific way of setting this up is to find a straight, lightly traveled section of road and measure your 0-60 (or 0-70, etc) mph time. The metering rod & jet combination that gives you the quickest time is your power step setting. Be careful as too high a terminal velocity may cause you problems with the local constabulary.

Once you have determined the best cruise and power settings, you need to put them together on the same metering rod & jet if you already haven’t done so. Calculate the flow areas for each case and find the combination that comes closest to them. It’s OK to go a little richer on the power step if it makes your selection easier. Drive your car and try out gradually stiffer metering rod springs. Pick the one that makes the car feel most responsive. Remember, the stiffer the spring, the more fuel your car will consume. If your car has a stumble on acceleration, gradually increase the pump shot.

Once you've set the main metering system’s jets and rods, reconnect the secondaries and go back to that section of road you were using before. Adjust the secondary jets until you have the best time. Make sure you do your testing when traffic is minimal.

I hope you find this helpful. You might find a better procedure for setting up your carb but I think what I’ve suggested is probably a good starting point. If you do find a better procedure, please let me know!