Carter 3-step metering rods and corresponding jets/springs

[Dragon Slayer]

Yes standard 120-380 jet in the secondary. AFB/AVS doesn't matter for secondary. No rod.

 

[AR67GTX]

I lost track of all the jet and rod sizes. Just realized that tall .101 jet is not much bigger than the primary in the FSM - just .003”. If I use the 164 rods that is just a hair richer which with .080 in the secondaries seems fine and balanced.

What do you think George?

 

[Dragon Slayer]

I don't see an issue trying that. G

 

[RJRENTON]

I’m now thinking these are pistons for the 3 step rods. Seem to fit a description George provided with a small washer at the top. Can you verify?View attachment 1249311

CORRECT....The bore in the air horn has a step in it to accomodate the washer under the metering rod retainer clip. I believe the purpose of the 3 step metering rod and corresponding piston is to provide a faster reaction time for the metering rod assembly to falling vacuum to provide better mixture control. Just my opinion of course.
BOB RENTON

 

[RJRENTON]

Here is a pic of the bleeds to check for clearance. The economizer is hidden as I mentioned, check it first with the .492, then you can get the larger bleed checked but do not go deep or you open the economizer and defeat it's purpose.

View attachment 1249344

EXCELLENT INFORMATION for future reference. The economizer orifice is internal to the booster venturii assemblies and is the first point where air and fuel is mixed. I believe these fuel and air flow orifices were calibrated by the origional carbs application via flow bench studies by Carter and Chrysler. Most users do not know these calibration points exist or their function.
BOB RENTON

 

[Dave6T4]

Do your pistons look like the ones I posted this morning? I would appreciate if you could check sometime.

Thanks

Yes, my pistons are same as picture. Edelbrock sells a spring kit that allows you to fine tune your metering rod action.

 

[Dragon Slayer]

CORRECT....The bore in the air horn has a step in it to accommodate the washer under the metering rod retainer clip. I believe the purpose of the 3 step metering rod and corresponding piston is to provide a faster reaction time for the metering rod assembly to falling vacuum to provide better mixture control. Just my opinion of course.
BOB RENTON

I had not paid much attention to those pistons, but just looked at my 4327. It is effectively a 2 stage piston. It is not the top washer, but the bottom washer that engages recess edge in bore and adds the additional spring pressure as the piston lowers. You get about 6 to 7/32" of travel down against the normal 604 spring under the piston. The remaining 5 to 6/32" of motion down to the economy position has to compress the bottom 604 and the spring on the piston. As that bottom washer stop on bore ledge. So yes a stronger movement as idle vacuum drops on acceleration, but also requires a strong vacuum signal to be on economy at idle to begin with. So motor with cam, might not have a good enough signal to get on the economy step.

 

[AR67GTX]

I need to take a closer look at those. I didn’t realize it’s a 2 piece piston (piston within a piston) if I understand correctly. I need to be sure they are moving smoothly and not gummed up.

Edit: Could use cleaning of soot but move freely.

 

[Dragon Slayer]

Not 2-piece piston, but the spring on piston between washers is the added vacuum spring. If you put piston in top with top off carb (without the bottom spring), you will see how the piston lower washer stops on the internal ridge. The piston won't bottom in top unless you pull on the rod. A normal piston like your hemi, if you dropped the piston in without spring it would bottom. It is effectively a 2 stage spring set up.

 

[AR67GTX]

Yes, after I took at closer look at it I realized the only moveable piece is the washer. Mine are a little smoked up but seem good otherwise. Dave is sending me another set so I’ll use the best of the 4. It has orange springs in it so I should be good with the tall jets and 3 stage rods.

Took the carb further apart yesterday and checked all the bleeds I could find. Didn’t try to get a particular size pin drill down them - just picked the largest one that would fit the opening and ran it down by hand. A couple little hang ups but for the most part they were clean and open.

thanks

 

[Don Frelier]

I have the original 3 step rods, primary jets and caps from my 66 AFB for the 383.
Rods : 16-404
Jets:. 120-252 not sure what the opening is?
They are the taller style.
Just let me know if you need them.

 

[AR67GTX]

Thanks, I think I have everything coming my way now that I will need. The jets I found are .101” or about .003” larger than stock but with ethanol I think that very slight enrichment will probably work out well. But, I’ll keep your offer in mind.

thanks again.

 

[Geoff 2]

Was unaware of AFB/AVS carbs having an internal ridge/step that is used with the piston that has the extra spring & washer. Which carbs used this set up? Any pics of the ridge in the piston bore? Thanks.

 

[AR67GTX]

I’m unsure about that but I plan to reassemble mine this morning and I’ll see if I can get a picture in there.

 

[Dragon Slayer]

Was unaware of AFB/AVS carbs having an internal ridge/step that is used with the piston that has the extra spring & washer. Which carbs used this set up? Any pics of the ridge in the piston bore? Thanks.

Any carb that used this piston. Go back to a few post and read them, I gave the part number. I know the 4131 used them, plus this AFB. It is not the most common piston, but it was used on some models. I never paid much attention to it, until this post than I examined my carb. It is the only way the extra spring would come into play.

 

[AR67GTX]

There is a seat there in my AFB. I don’t have another model AFB apart right now for comparison.

 

[RJRENTON]

Any carb that used this piston. Go back to a few post and read them, I gave the part number. I know the 4131 used them, plus this AFB. It is not the most common piston, but it was used on some models. I never paid much attention to it, until this post than I examined my carb. It is the only way the extra spring would come into play.

The Carter AFB #s 3854S, 4294S used on the 273 HP applications plus first generation AVS used on 340 #4424S & 4425S used a 3 step up rod. And 3871S used on certain 413 applications. I'm not sure if the Carter/GM or Carter/AMC applications used the three step piston and metering rods. I believe the three step metering yielded a smoother transition from cruise to power, perhaps increasing fuel economy or driver's consciousness. Just my opinion of course.
BOB RENTON

 

[RJRENTON]

There is a seat there in my AFB. I don’t have another model AFB apart right now for comparison.View attachment 1257338

EXCELLENT INFORMATION and photo of the step up piston bore.....
BOB RENTON

 

[Geoff 2]

AR, yes, thanks for the pic.

 

[Dragon Slayer]

The Carter AFB #s 3854S, 4294S used on the 273 HP applications plus first generation AVS used on 340 #4424S & 4425S used a 3 step up rod. And 3871S used on certain 413 applications. I'm not sure if the Carter/GM or Carter/AMC applications used the three step piston and metering rods. I believe the three step metering yielded a smoother transition from cruise to power, perhaps increasing fuel economy or driver's consciousness. Just my opinion of course.
BOB RENTON

It actually has nothing to do with the 3 step rod. Plenty of carter 3 step rod carburetors use the standard AFB 2 step rod piston. There are a handful of carbs using this style piston with added spring. 4131 is one besides these 440 afb from 67. I do not believe I have seen an AVS with 3 step rods use this piston. But there could be a model that does.