Carter AFB metering tips

This thread in response to another thread asking questions about Carter AFB metering rods and jets.

This thread will consider ONLY those AFB carburetors made by Carter BEFORE they were purchased by Federal Mogul (approximately 1984), and does not address the AFB clones regardless of what name (even Carter) that may be on the clone.

Carter produced 505 different type AFB carburetors beginning in 1957 up until their purchase by Federal Mogul. As I have custody of the remaining Carter prints, I do have access to the original specifications for most of these carburetors. NO, I will NOT publish these specification on the internet! The reason being that access to the specifications would HURT more enthusiasts than it would help!

Why???

Regardless of what you may or may not have read on other websites; THESE PARTS ARE NOT NECESSARILY INTERCHANGEABLE!!!!!

Main metering jets: So far I have identified three separate and distinct physical types of jets used in the AFB. While the thread size is the same, the metering area shape in the jets is different, and they should not be interchanged among types.

Step-up rods: So far I have identified 19 different profiles!!! Yes, that is not a misprint; 19 different profiles. I have identified 381 different step-up rods (but I may have missed a couple)

How are profiles different?

Say you have two totally different rods, each with a high vacuum step of 0.068, and a power step of 0.048. Possibly the power step on the first is 0.250 long where on the second, the power step is 0.400 long. The second rod would run exceptionally rich at mid-range if substituted for the first rod.

One other comment on the rods: comparing the difference in diameter from the high vacuum step to the low vacuum step from rod to rod only proves you can subtract; it means little in comparing the rods. Example: a rod with a high vacuum step of 0.068 and a low vacuum step of 0.054 would have a difference of 0.014. Another rod with a high vacuum step of 0.060 and a low vacuum step of 0.046 would also have a difference of 0.014. What does this difference mean? Nothing! The metering comparison should be to calculate the area of the primary set and subtract the dynamic area (that portion of the step-up rod in the jet at any time) of the step-up rod AND THEN COMPARE AREA DIFFERENCES!

So what rods ARE interchangeable??? Consult your original factory shop manual. The carburetor section should have a listing of rods (standard, 1 size lean, 2 sizes lean and 1 size rich assuming all of these were offered) with the original PART numbers. These will be interchangeable in your carburetor.

Generally, Carter used the following calibrations:

Sea level ~ 4000 feet - standard

4001 ~ 6000 feet - 1 size lean

6001 ~ 7000 feet - 2 sizes lean

7001 ~ up - generally not specified. Use the 2 size lean rod

Some truck carbs were assigned 1 size rich rods if the truck were to be used for heavy duty.

Another question I am often asked - May the clone rods and jets be used successfully in genuine Carter AFB carbs? Well you have a 1 in 3 probability for the jets, and a 1 in 19 probability for the rods. Its your engine!

And finally, if in doubt, it is better to be too rich than too lean. Too rich wastes power and fuel. Too lean wastes engines!

Jon.