3-step metering rods? High jets?

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1974DartSwinger

1974DartSwinger

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360 with a 4bbl Carter AFB . . .
My carter strip kit from e-bay and the "bonus" eddy 600 I snagged for cheap are on their way. I was reading up in my carter tuning / rebuilding book to get a feel for things before going nuts. In the "fine print" on one picture they said that AFB carbs with dimpled metering rod covers have a 3-step metering rod installed and for these rods you have to use high jets. I haven't done anything to this carb other than tweak the idle mixture and fix the choke (bad pull off).

Is there anything I should know about this that could mess up my carb? I was thinking that I could just figure out what size "high jets" are in there now and install the same size normal ones from the strip kit then go on my merry way tuning with the regular 2-step metering rods in the kit. I figured I could also use the flat top metering rod covers from the eddy carb or buy some regular carter ones.
Am I mistaken?
I also picked up a used eddy air fuel gauge to help in the fine tuning process. Is this "narrow band" a/f gauge fine for my needs? I didn't feel like dropping $250 on a LM-1 wide band set up just yet.
Thanks!!
 
Why would you go with the coarser 2 step rods? You can if you wish by doing what you say. You have to change everything in the system. Jets, Rods, springs, cover plates. I'd calculate the open area at Idle. ID of the original jets - largest OD of original rods and match up a similar open area with the new jets and rods, to start. Good Luck
 
I thought the AFB rods had two steps, the AVS had three, and the Edelbrocks all have two... So if you have a Carter strip kit, it may have 2 or three depending and the corresponding jet, and Edelbrock's has the two step with matching jets. The tall jet IIRC looks more like a holley jet rather than a flat Edelbrock jet. It's been a while since I took apart an AVS.
 
First, the 2-step rods and jets are just fine, and much more available.

As far as 3-step rods and long jets, they were common to the AVS carbs, but fitted to some OEM AFB's.

What carb # are you working on?
 
Its a Carter AFB #4120. Its the factory HiPo 273 carb. I'm running the AFB with the original 273 4bbl intake ontop of a '74 360 block and heads.

Can I run the 2-step rods with the long jets?
 
I do not believe that combination will work properly. You would be better to convert to later Edelbrock jets and 2-step rods which are commonly available, unless you have scored a complete 3-step/long jet Strip Kit.
 
Is the metering rod piston the same with this carb? Meaning, I can just put in the 2-step rods / jets without any problems?
 
1974DartSwinger said:
Its a Carter AFB #4120. Its the factory HiPo 273 carb. I'm running the AFB with the original 273 4bbl intake ontop of a '74 360 block and heads.

Can I run the 2-step rods with the long jets?
Click to expand...

I would not run 2 step rods with the tall jets. If you have an original 4120s AFB sell it, as they are worth good money and just use the Edelbrock AFB. There are probably more than a few people here who would love to get an Original 66 automatic 273 AFB. I'd run both before I sold the 4120s first.

Most Original Mopar AFB's and AVS carbs came with the 3 step system for finer fuel metering, Idle, Midrange, and our favorite, Wide Open Throttle. Chevy carbs and most aftermarket AFB and their clones came with the 2 step system. I think some early Premium lines like Cadillac, Oldsmobile and Pontiac may also have the 3 step system.
 
The advantage to running the two step rods/jets/flat caps is you can easily buy the Edelbrock stuff cheap and drop it in. That is a huge advantage. Yes the two step rods will work on the three step pistons. But you have to run two step rods with two step jets with the flat caps. For the secondary side just screw the Edelbrock jets in and go.
 
The three step rods were used from the factory to help meet emission standards. It was able to lean out the A/F more while still allowing light part throttle use on the 2 step.

If your Carter kit has 3 step rods sell them on Ebay to those stuck in a state that requires smog tests for those old cars. Expect for maybe getting a bit better MPG with a 3 step rod there is no reason not to use the 2 step rods and jets.
 
dodge freak said:
The three step rods were used from the factory to help meet emission standards. It was able to lean out the A/F more while still allowing light part throttle use on the 2 step.

If your Carter kit has 3 step rods sell them on Ebay to those stuck in a state that requires smog tests for those old cars. Expect for maybe getting a bit better MPG with a 3 step rod there is no reason not to use the 2 step rods and jets.
Click to expand...

If this is true why did they come on 67 down cars, before emissions? I would not waste time changing out your 3 step rods, jets and covers unless you have a problem or non standard setup.

340sFastback "The advantage to running the two step rods/jets/flat caps is you can easily buy the Edelbrock stuff cheap and drop it in. That is a huge advantage. Yes the two step rods will work on the three step pistons. But you have to run two step rods with two step jets with the flat caps. For the secondary side just screw the Edelbrock jets in and go. "

Agreed.
 
Well basically I think its running a bit lean at WOT, theres a bit of a flat spot above 1/2 throttle but that has gotten somewhat better since putting in the mopar electronic ignition and playing with my timing. There is still a very slight surge while cruising.

The set up is the 4120 AFB carb with matching 273 hipo intake ontop of a 1974 360 block and heads. I'm almost certain the jets / rods in the carb are original for the 273. It makes me think that my cruising lean surge, slight flat spot above 1/2 are all just from being a tad lean for the 360. I might not even have to play with the metering rods, maybe just put in a size larger or two set of jets may cure everything. We shall see.
My air fuel gauge I ordered should be here soon so that will give me a for sure answer of whats going on.
 
1974DartSwinger said:
Well basically I think its running a bit lean at WOT, theres a bit of a flat spot above 1/2 throttle but that has gotten somewhat better since putting in the mopar electronic ignition and playing with my timing. There is still a very slight surge while cruising.

The set up is the 4120 AFB carb with matching 273 hipo intake ontop of a 1974 360 block and heads. I'm almost certain the jets / rods in the carb are original for the 273. It makes me think that my cruising lean surge, slight flat spot above 1/2 are all just from being a tad lean for the 360. I might not even have to play with the metering rods, maybe just put in a size larger or two set of jets may cure everything. We shall see.
My air fuel gauge I ordered should be here soon so that will give me a for sure answer of whats going on.
Click to expand...

It does indeed sound like you are a bit lean.
 
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