Assuming I got a matching exhaust manifold and did the swap, what would be the recommended routing of heater hoses for this manifold? Would I just interrupt the hose that comes out of the thermostat housing?

Seems to me that there would be some loss of rigidity or structural integrity to the manifolds if they are not bolted together. Having them hanging off the side of the block individually seems like it would cause them to move up and down more easily. Did the factory cars with the later 258s that used independent manifolds (like the 2 barrel one I am trying to install) come with some kind of support bracket or brackets? If so, could I grab it off a Jeep with a 4.0 from the junkyard?

258 1bbl was rated at 110hp.

258 2bbl was rated at 115-120hp.

We're talking the difference between 175 CFM and 195 CFM. You're also going from a really simple carburetor (YF) to a more complex and notorious for trouble carburetor (BBD). Is it really worth it for 5-10 horsepower? Your air conditioning or power steering pull that much power.

Plus you should change the exhaust manifold with it and should find a way to wire up the 2bbl intake's electric manifold heater and water heating apparatus because unlike the 1bbl intake that has a built-in heat stove, those are the 2bbl intake's only methods of improved fuel vaporization for cold weather.

It also has "121" stamped into the side of the float body on the same side as the throttle lever. Is this carb too big for my 258? 

1) original exhaust manifold had a BAD exhaust leak, blown out gasket, so had to come out anyway.

2) original manifolds were rather ugly, nothing but rust colour, so would have to be refinished, blasted/painted, etc,  and one stripped out thread for exhaust pipe stud to be repaired.

3) Weight, not in the "performance" sense, but lifting those two joined cast iron monsters out of the engine bay was a piece of work! (I'm not getting any younger!). Much easier installing/dealing with separate manifolds, and the aluminum intake is light as a feather!

4)Exhaust is in the same location, no more donut gasket to deal with.

5)Aluminum intake has three threaded ports for CTO, or aftermarket temp gauges or whatever. Options!

6)better flowing (I'm told) exhaust manifold, hey, 2 or 3 hp is nothing to sneeze at!

7)still an all AMC setup, best they had for the 258.

8) Looks good! Runs good! Bolts up! Is life good or what!

ok, still with me? Couple of things to watch out for:

1)Power steering bracket from the cast iron manifold will need to be modified, or better yet get the one from the aluminum manifold motors. The original will NOT work as is.

2)Throttle mounting bracket is VERY different, again, get the one from the alum. motors.

3)Choke will have to be switched to electric, or to stay old school, get an aftermarket heat choke kit, which I did. Yes!, still available out there.

4)Carb throttle rod may need to be replaced, again, with the late model one.

5)heat stove from the cast iron exhaust manifold will not work on the divorced manifold, and is probably rusted to heck anyway. This was, for me, the hardest piece to find, a good condition 81+ heat stove, but I did! ( I know, Uncljohn, you don't need these, but i do! :)

So, you see, a "simple" bolt on can have it's challenges. Mr mantonas, good luck, another option not discussed here, is to replace the old 1bbl cast iron manifold with a cast iron 2bbl manifold, which would take most of the above issues out of the picture.

Or, rebuild the YF, and motor on into the sunset!

good luck, gz

Again, just trying to make the OP aware of what is involved. Imo, he would be better of just rebuilding his YF, kit is about $25., and have no fabrication issues or real down time on the car.

Having said that, a Pacer was/is a heavy car, and any kind of power gain is a good thing.

thank you, gz

That damn power steering bracket issue has jumped up and bit me in the rear end but good. The rear bracket would not clear the newer style exhaust manifold. I found one on eBay that looked like it would clear, and I bought it for a painfully high amount of money, and it did clear. Unfortunately, it does not let the pump rotate the way it needs to in order to loosen and tighten the belt. I'm stumped now. I'm seriously considering going back to the one barrel setup and maybe selling all this two barrel stuff.

The only thing that's nagging me is that I might just be one bracket away from getting this to work; the aluminum two-piece bracket that fits around the pump itself; maybe there's one that will work. Does anyone know if this is true?

I don't know if this is going to work, but I'm going to put in some eBay links to help explain this. 

Here is a link to a bracket on eBay that I think is the one that I need. Notice the way the grooved slot on the left side has a scalloped upper surface:

http://i.ebayimg.com/images/g/bYcAAOSwTA9X6yNH/s-l1600.jpg

Now here is one that I think is like the one I have. Notice the fully built-up edge of the slot on the bottom left of the middle photo:

http://www.ebay.com/itm/192163065920

Here are a few complete pump assemblies on eBay with all brackets where if you look at the photos you can see the clearance and how it works:

http://www.ebay.com/itm/Jeep-cj-wrangler-PS-pump-and-Brackets-amc-258-cu-in-4-2L-76-91-/182574254239?hash=item2a8246089f:g:IHgAAOSwhvFZGNKk&vxp=mtr

http://www.ebay.com/itm/401331002589

http://www.ebay.com/itm/112421032182

So I think I have this solved. What I would really like to do is get a part number for this bracket so I can do some more thorough searches for purchasing this part. I think $75 for a bracket or $175 (or more) for a used power steering pump assembly is a little steep; there's gotta be some of these sitting around in a junk yard or somebody's garage. How do I get access to a parts manual for an 80s AMC or Jeep CJ?

I bolted down my 80s BBD 2 barrel carb that I bought on eBay for about 30 bucks, hooked up the fuel line to a gas can, and started it up, and it ran great! I still need to work on the throttle bracket, and I won't know for sure until I put it under load by actually driving it, but it seems as thought the stepper motor might have been left in a decent position for driveability. 

One issue I have is numerous exhaust leaks from all the plugs in the exhaust manifold for the EGR connection and air injection ports. Access is so limited I might have to temporarily remove the carburetor to get to them to retighten. Oh well.

i also wrapped my manifold and downpipe with fiberglass cloth strip i got from summit. wow, does that make overall underhood and cabin temps lower! i laced it up with stainless safety wire, and had a couple of minor loose spots i sealed with red silicone. (you can touch the manifold of the engine after a long hot run. its "hot" but not emergency room time :-)

Subject change: I bought one of those tiny 4" round air cleaners and the hood won't close. There's a spacer under the carb that came with it when I bought it on eBay. I assumed it was part of the factory configuration and needed to be there, either so the butterflies would clear the inside of the manifold or to help vaporize the fuel before it is forced to make an abrupt right angle turn. Even if neither of these is true, I would prefer to keep it because I think a spacer helps low end throttle response. However, if I don't absolutely need it, and I can close the hood if I remove it, I'm removing it. Can I get by without it?

P.S. I have a 4" round air cleaner for sale :)

I wired up all the manifold electrics just like the 83+ TSM shows and it runs great in the winter. I could tell the difference when the electric heater relay died the winter before last. I used a Cherokee plastic heater valve to replace the Pacer's original so the coolant will continue to circulate like it's supposed to.

After I did that, the ball stud on the throttle lever of the BBD carburetor was not in anywhere near the right place for the rod attached to the throttle linkage to be able to pull it down. So I fabricated a right angle bracket out of some stainless steel I had lying around to move it from the side to the rear of the carburetor. 

Also, the original rod was not the right length and the connection to the ball stud was 90 degrees off where it should have been. So I used two of these and some threaded rod:

https://www.summitracing.com/search/brand/qa1-precision-products/product-line/qa1-throttle-linkage-quick-disconnect-couplers?autoview=SKU&ibanner=SREPD5

So after that, everything worked as far as the carburetor installation was concerned. 

Also, as it turned out, I had a factory 2 barrel air cleaner among the storage unit full of Pacer parts that I have that I bought from a guy a couple of years ago. It fit on the BBD 2 barrel perfectly!

So I am going to consider this task complete, after over a year of struggles. I have done a lot of other stuff on this car, and I will definitely be doing more, including an MC2100 carburetor installation at some point. When I do that, I may revive this thread, or start a new one, I don't know. But I really appreciate everyone's help, suggestions, and interest!