Engine How to convert carb from a 1 barrel carb to a 2 barrel carb.

[rothsteinbarry]

I'm looking to convert my 1977 Jeep CJ7 AMC 258 i6 / 4.2l liter - carb from a 1 barrel carb to a 2 barrel carb.

Any suggestion would be greatly appreciated. I have found online the following carb online and would like to know if anyone had success with the following Holley Carb:

500 CFM Holley 2300 2-Barrel Carburetor | Holley 0-4412S

 

[TSB8C]

If looking to keep the same 1-bbl intake, you'll need an adaptor (not ideal at all, but doable). Here's an example: Adaptor

Better off to swap the intake for a newer 2-bbl and skip the adaptor.

 

[Black Thumb]

The AMC 258 i6 / 4.2l needs a relatively small (1.08) venturi because it is a low RPM high torque engine. That carb says "high flow" venturi but doesn't say what it is. I think that carb would be better on the AMC 304 or larger V8, and maybe on the 258 if you have upgraded cams, headers, and a different intake to try and run at higher RPMs. You might get some more top end with it if your engine is modified, but it's going to be harder to tune, and you're probably going to sacrifice low end (what a CJ7 is) and some drivability. If your engine is unmodified I would defiantly go to with the smaller 350 Holley. It will be way easier to get it running right, as it's much more matched to the engine. It also won't sacrifice your low end, which is where the 258 shines and is it's strong point. The 258 really doesn't have much of a top end anyway, and you just can't buy it with a carburetor.

Most swaps (the easiest and most effective) to a two barrel on the AMC 258 are to the Motor craft 2100 (1.08 Venturi), which is almost already set up to have optimal combo for that engine. It will be alot more plug and play. It does require an $80 or so adaptor plate, but it fits up easy. (Link)-> The Ultimate Guide to the Motorcraft 2100 Carburetor -<.

I'm not saying you couldn't make it work with a lot of tuning, jetting, and playing with the fuel pump (probably can), but it really doesn't go with that engine. You can only wring out so much performance. What you are buying with the 2100 is faster/smoother throttle response, and a more efficient burn (fuel/air) mix. With carbs there is really only kind of an optimal. I sort of think it shouldn't be the primary focus if you are trying to get more performance out of an engine, but kind of the last place you go once you have made performance mods, and need to match those with air/fuel.

I have the same engine with the same Carter Carburetor, and I have been researching the heck out of it as well. The right one from Holley is the 350 with the 1.08 Venturi. Edelbrock doesn't make one small enough. The right OEM one is Motor craft 2100 1.08 Venturi. Both are pretty much ideal for an unmodified AMC 258, and will produce noticeable improvements with the least fuss and frustration.

 

[Black Thumb]

The issue is that a carburetor is part of a system. All the parts of that system work together, and changing one part doesn't have that much impact on the system. If you want to see a significant difference in power and response you really need to change your intake as well as the carburetor. On a CJ, and most old inline 6s, the stock manifold is actually part of the headers. So you need new headers as well. There isn't a performance manifold that has combined intake/exhaust like many of the old inline 6s. I'm not actually sure how well the stock set up really works in the cold either, and it definitely robs the engine of power when it isn't freezing, but there were a whole bunch of inline engines that did it that way before the '80s. Nevertheless, even though it's the same engine you end up changing everything from the air coming in to the air going out.

There is a full kit (with headers) from Clifford that pairs a Webber 38/38 (would be better with a 32/36) with a single plane intake. They're saying this is specifically for a Jeep with an AMC258, but I'm not seeing it.

6=8J258 Jeep/AMC 232/258 Combination 1972/90

Add 70% Power & Torque just bolting on 6=8 Combination. Carb come to you ready to run and you can tune off idle making this combination far superior. When you can't tune off idle, you have a mess. This is our 6=8 Combination parts list: 57-2300 Dual Outlet Headers/Gasket/Street Hookup Kit/Header...

cliffordperformance.net

I honestly don't like this one, because it's an open plane (drag race style) intake AND it's a synchronous carb. At least the 32/36 would mellow out the throttle in that huge open plenum when driving around. Both of these reduce your low end torque and reduce drivability. They claim 70% improvement, but most of that is going to be top end. I guess of you want a "fast" antique Jeep this is good, but if you want it torquey and good on the low end for crawling slowly along a rough trail this is bad. Old Jeeps are already a pretty rough ride, and making them behave even more poorly on the street doesn't seem all that wise to me... Seems dumb to me to put race stuff on a low HP high torque AMC 258 i6 / 4.2l ... Clifford used to make a sort-of-a dual-plane (more a divided plenum than a true dual plane) direct bolt for a double barrel carburetor, but they quit making them, and now they're as rare as hen's teeth on Ebay, but they do occasionally show up I am told.


The only intake I can find new that really seems like it would be a desirable upgrade is the Offenhauser performance 5999dp (dual plane) intake that will direct bolt the Holley 350 or a 2100 with no adaptor plate (no restrictions).
https://www.summitracing.com/parts/ofy-5999dp
Seems like that would work best with with Holley 350 or the 2100 to provide more power, torque, and faster throttle response while still behaving well on the street.
You still need new headers and some light fab to mate them to your exhaust, or rather new exhaust.
Cheapest headers I could find for it:

https://www.summitracing.com/parts/hed-99600

Then you need
✔ Gaskets
✔ Throttle linkage
✔ Fuel line + filter
✔ Air cleaner
✔ Exhaust hookup
probably a flow thru muffler...

So, realistically for good parts and a clean install with everything you are going to need it's probably $2k on the cheap and $2.5K-$3K for the top of the line stuff...

For all that you are probably, realistically, getting 30-40% more horsepower, and probably 30 more pounds of torque. But, you are getting it earlier and much stronger than it comes on with the stock set up. I think it's almost worth it for throttle response. This is the way I'm leaning because I want maximum trail/beach capable, but mostly because I want to keep it naturally aspirated.

I'm sure lots of guys on the board can tell you about EFI upgrades. As involved as changing the whole intake and exhaust is, there is more with EFI and it's usually more expensive. That said, people seem to love them. I actually engine swaped a F5 Cobra from an 5.0 EFI to a 427 stroked small block with a QuickFuel 750. No replacement for displacement.

If you look at engine swaps as an alternative, both are actually a pretty good deal cost wise, and less work. Or, at least that's the way it seems to me. I'm sure there are more knowledgeable people with different opinions...