1971 Holley list numbers are 4801 for LT-1s, 4802 for LS6 automatics, and 4803 for LS6 4-speeds. Each has a different GM 7-digit part number. However, every spec that I find from Holley is the same for all three. Early metering blocks were all the same, as well as the second style metering blocks. The plastic fast idle cam,main body, etc all have the same casting and mold numbers. I can’t find any differences between the three carbs. Were there any? Why different part numbers and list numbers if there were not? Any help understanding this would be appreciated.

 

My factory 70 LT-1 OEM carb had adjustable idle air bleeds with an almost invisible screw (sealed under putty) in front of the choke horn (looks like a dimple). They were there to fine tune the emission and idle settings, which would almost certainly change, even from an auto to a 4spd car, and absolutely with cubic inches. I assume that all the other 70-72 Holleys had those as well due to emissions. I have seen a few with them so I know that they were OEM carbs. Since they were factory sealed I assume that is at least one reason Holley would need a new part number, even if the rest is identical. (IIRC I do not remember the jets and metering block numbers being exactly the same though.) I currently have a 4803 for my LS6 but it is not an OEM installed carb, it is an over the counter one ('78) and it does not have this adjustable idle air bleed, it has a brass fitting installed in the idle air bleed as is standard Holley practice. If you look closely the OEM ones are missing the brass bleed insert. Just as well, I had major idle problems with my LT-1 and those things were the culprit til we got them re-adjusted.

So even if the main body casting was identical, some carbs were drilled for these extra screws, and some were not. Some had brass inserts, and some did not. The brass inserts could be different sizes. And unless you measure the brass inserts or the adjustments.....