Putting 80s 258 2 barrel intake on 76 engine - wha

I've run the 258 for the last 15 years on 2 different 1980 Spirits with the bbd carburetor and frankly if you have any skill at all in rebuilding carburetors the BBD works quite well. If it is a simple decision of using available parts then there is nothing wrong with them other than one thing, at 193CFM it is way too small to satisfactorily develop the HP that an engine the size of the 258 can develop.
But even that said mine with an Isky 256 cam developed 110HP at the rear wheels, dyno measured.
As to Isky specs, it is a company that has been around for a long time as has the cam and their specs are what they are and haven't changed since I have been using them. But the key is you need to be good with working with Carburetors, something that has become a lost art. And if you aren't good with a carburetor you should not be messing with any of them.
As to emissions, AMC did no one any favors as to how they are covered in their manuals, it takes a bit of work to make them correct after all the years and the ill contrived modifications the inept have made to them to "improve" how they work.
Both my 258's have been or are factory correct as to emissions and run fine. The BBD's reputation has been largely tarnished by the fact it has been on an engine with about 22 feet of vacuum lines running assorted and poorly documented directions.
I have no use for the largely used motocraft 2bbl carburetor, it is a lot of work and expense to get something that is not that much larger. If I am going to change the carburetor on a 258 I go straight to a Carter 400 CFM AFB or an Edelbrock 500 CFM AFB and make good use of a cam change. Yes the motocraft carburetor is simpler than a BBD but not much larger. At least not much to make it worth the effort of getting one on the engine.
And if simplicity is what you want because a BBD needs some skill working with Carburetors and you have the money and the parts FI is available. But for pure BOLT ON updating, a BBD is a Cheap 15 HP.

One thing I forgot to mention is that the discussion in this forum is making me wonder whether I should just stick with the one barrel. I really had no plans to do this; originally I wanted to do as little as possible with the engine then someday drop in a complete 4.0 from a 90s Cherokee or Grand Cherokee. Somehow I ended up doing this two barrel conversion. If I have to put in a cam to really reap the benefits of a bigger carb, why bother? The bad thing is, to reuse my existing manifolds I need to get a machine shop to remove the nuts and bolts that connect the intake and exhaust manifolds together, since they are seized. And I will have to rebuild the YF, but it looks like it might be the world's simplest carburetor so that shouldn't be a big deal.

I bought a beautiful, clean, apparently brand new Motorcraft 2100 or 2150 carburetor on eBay for 15 bucks a little while ago. It has an aluminum tag with what appears to be a simplified version of the AMC logo on it, consisting of a triangle on the left and a rectangle on the right with a space between them, and below those symbols "4RHA2." There is also a sticker on a heat shield plate in front of the choke coil with a sticker on it that also has "4RHA2" on it, with "5 10 86" above it (looks like a date) and "E4FE AA" below it. I was going to go with this first but then I decided I didn't want to mess with the adapter plate and the linkage setup. But now that I'm going to have a project centered around making linkages work, why not go with it.

The engine's volumetric efficiency is a measurable value and with a correctly matched carburetor you will receive the best performance possible out of your engine. The formula for calculating how much CFM (cubic feet per minute) an engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.

The formula depends on a reasonable estimate of an rpm that would be achievable and volumetric efficiency which for the most part is a guesstament and can be ignored.
A tachometer is a fair enough tool to determine rpm, lacking that 4500 or 5000 rpm is a reasonable estimate of what rpm might be seen.
Plugging those numbers back into the equation you get:
(258 X 5000)/3456 = 373.4 CFM
Considering that a Carter BBD carburetor is 193 CFM and a Carter YF is generally considered to be about 175 CFM
The rpm range for those carburetors can be calculated to be
2582rpm for the bbd and 2345rpm for the Yf it bgins to get a little understandable why those engines fall flat at a very low rpm.
The formula does not take into account the number of cylinders or whether it is modified or not.
So no, the MC2100 does not come anywhere near too big.
When I build an I6 engine for the street I will generally by pass any 2bbl carburetor at all although you can get up to 500 cfm in a Holly and go straight for a Carter 400 CFM AFB and lacking the ability to find one an Edelbrock 500 CFM AFB.
A Small four barrel which will run on the primaries most of the time has better throttle response at lower rpms than the same size 2bbl carburetor.
In general, the AMC I-6 engine is both under carbureted and for the most part the cam can be considered very weak also. They are configured to give the illusion of performance in city traffic at best.
How does this compare to a generally accepted V8 engine
A 304 is happy with something around 440 CFM, a 360 with something about 500 CFM and a 401 with something about 580 CFM carb.
Rpm expected to be seen as part of the equation will affect selection as will available carburetor size.
Often you will see a 600 or 650 used for the 300 and 350 Cub in sized engines and 700 or 750 CFM carbs used on 390's and 401's
I use a 625 CFM AFB on my 390 and will probably use the same carburetor on a 360.
At certain levels of performance the efficiency of the intake system becomes important but for the most part it can be ignored as not applicable.
In general a V8 engine is out fitted with a carburetor larger than the math indicates is optimum and it makes no sense that an I6 engine be treated any different if for no other reason than it limits the potential that might be utilized in time

mantonas wrote: One thing I forgot to mention is that the discussion in this forum is making me wonder whether I should just stick with the one barrel. I really had no plans to do this; originally I wanted to do as little as possible with the engine then someday drop in a complete 4.0 from a 90s Cherokee or Grand Cherokee. Somehow I ended up doing this two barrel conversion. If I have to put in a cam to really reap the benefits of a bigger carb, why bother? The bad thing is, to reuse my existing manifolds I need to get a machine shop to remove the nuts and bolts that connect the intake and exhaust manifolds together, since they are seized. And I will have to rebuild the YF, but it looks like it might be the world's simplest carburetor so that shouldn't be a big deal.

While I am an advocate of by passing the use of going through the expense and bother of using one of the autolite 2bbls off of an AMC V8 to replace the OEM carburetors either the single barrel Carter YF or the BBD Carter 2bbl for no other reason then the benefit received is not worth the value expended to do so in my opinion. They too are also undersized for the size of the engine so why bother. If you are satisfied with the performance you are getting there is nothing wrong with tuning what you have and calling it good to go.
I have had excellent luck with the BBD Carter 2bbl carb and the YF works well too. Neither of them are particularly difficult to tune and use. The BBD Carter 2bbl is tricky to rebuild, but by taking your time and following instructions they come out o.k., too.

As to the bolts and such sized holding the manifold together, heat applied to them has generally cause them to break free. A typical hand held propane torch has been adequate for me anyway as a supplier of heat. Once hot soaking things down with WD-40 has aided in disassembly. However that can be very smoky. An Alternative to WD-40 and similar products is bee's Wax, usually purchased in a small brown cube or some shape and held where you wanted it to be to cause it to melt.
The advantage? It is below the temperature that causes bee's wax to melt so no smoke. It doesn't hurt either to rap things with a small ball peen hammer which encourages things to break free. "Rap" not BEAT which encourages things bend and break. When you install the later intake manifold you will also want to use the exhaust manifold too.

A tip if you are going to re-install the OEM intake and exhaust. Hopefully you will be following the instructions in the TSM to do so, but remember the bolts and such that hold the two pieces together, do not tighten them very tight initially. When installed on the engine you want the two pieces to "center" themselves in the cylinder head and then tighten the mounting hardware that holds the two manifolds together. Done that way the potential of having a gasket blow out are minimized.