2bbl weber on offenhauser 4 bbl IM

2bbl weber on offenhauser 4 bbl IM
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83CJLaredo

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Sellersville Pennsylvania
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83' laredo 4.2 5 speed
Hey jeepers im new to this forum and figured i would ask a question finally that i really wanted to know the answer to. The previous owner of my cj put on a offy intake manifold and has a 4bbl edelbrock carb on it. Its just a stock AMC 258 i6 / 4.2l and i think that is overkill. Now when i pulled the 4bbl off the intake manifol has the 4 port holes in it, only the 2 facing the engine had carbon buildup and the 2 ports facing the drivers side fender were completely clean like it was brand new. The jeep doesnt run to bad, it idles and it just hesitates a little while at low rpm. Now i had a weber 2bbl on my yj and i loved it. i was kinda curious to see if i buy an adapter plate for 4bbl to 2bbl (Link Below) and put on a 2bbl weber if that was a good decision and if would actually run better, im just not sure about those 2 intake ports being clean like the other butterfly in the 4bbl edelbrock isnt opening. But like i said it runs and doesnt misfire it just hesitates in low rpm. Thanks!
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Hi - I don't have this setup currently, although I do have a 76 with a AMC 258 i6 / 4.2l . The 4bbl carb should be a better setup overall assuming the carb is operating properly. Usually these are a small 4 bbl like 450cfm or 500cfm. If the carb is too big, it may not flow right for the AMC 258 i6 / 4.2l .

Which holes have the carbon? The primary or secondary side?

Anyway, it is important to tune the carb for proper operation.

First - verify the ignition parts are good. Good cap, rotor, plugs, etc.... Make sure the ignition timing is set right and the vacuum advance is attached and operating. This will help eliminate any variables from the ignition.

Second - Check the plugs to see if they indicate the engine is running rich or lean. There are several books and online picture references. Generally black sooty plugs indicate too rich, and burnt white plugs are too lean. Hopefully you can find something in between. If this CJ has an oxygen sensor, you may be able to get a reading from that. The main jets can be changed out to adjust this.

Verify float level is correct.

Check needle and seat for proper seal. There shouldn't be any obvious wear.

Check accelerator pump and diaphragm. Operating the throttle should deliver a shot of fuel. This can affect your low rpm acceleration.

Check the idle screw adjustment.

Check the vacuum secondary operation. There should be a vacuum connection to the manifold. There should be a spring or adjustment for this.

There is also an adjustment for power enrichment. The edelbrock has needle valve adjustments. The Holley method uses a power valve. The important concept here is that the enrichment circuit is activated when a load is placed on the engine. It should not be active cruising down the road at high vacuum. I rigged up a vaccum gauge on the dash and a fuel mixture meter that I could monitor from the O2 sensor. Then I can tell what the vacuum is and when the carb is adding fuel. This is a bit tricky, but worth the effort once you get it all tuned in.

Sorry for the long explanation. Hope it helps you make a decision.
 
83CJLaredo said:
Cant get any info from anyone? i really need to get rid of this 4bbl so any input would be nice
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Seriously? It's been less than a day. I understand your angst and desire for an answer but that's not going to help your odds of getting a response from anyone around here.

First, what's the CFM rating of the 4 barrel carb that was on there? A 390 CFM 4 barrel carb isn't to big for a stock AMC 258 i6 / 4.2l , there are several members here who run them, one guys wife's CJ is running a AMC 258 i6 / 4.2l with a 390 CFM 4 barrel.

Second, the hesitation is either from a bad (faulty) or incorrectly sized power valve. Power valve rating should be 1/2 of manifold vacuum at idle, i.e. 17 in/Hg vacuum at idle you should use a 8.5 power valve.

Last, its your CJ, if you want to ditch the 4 barrel, ditch the 4 barrel. My advice to you is though, if you're going to ditch the 4 barrel, ditch the 4 barrel manifold as well. Weber carbs are temperamental at best on a 2 barrel intake, I don't think its young to run any better on a 4 barrel intake.
 
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My suggestion to questions like this is to go with the mc2100, if you wanna ditch the 4 bbl. webers are a pia in my book you can't beat the simplicity and reliability of the ole mc2100, plus you can pick up parts for em just about anywhere, and you'll save yourself about $AMC 150 in the process. I'm with elwood on ditching the offy intake as well, go back with a factory aluminum 2bbl intake with a 2bbl carb, it most likely wont run as good as It could with a 2bbl-4bbl adapter.

Droooiiddd 2
 
The problem with the stock AMC 258 i6 / 4.2l is head flow. a 390cfm should be about right...anything above that will show poor performance. I ran a 500cfm Edelbrock on an extremely built AMC 258 i6 / 4.2l /4.0 hybrid and it was perfect. Bigger carb doesn't mean better if the engine cant handle the increased flow and doesn't have the valve duration needed to accommodate it.

So verify for us what CFM rating your carb is.

The Weber is a touchy animal. I've ran them on 3 motors and they tend to need special attention to jetting. They are usually over jetted. Once that problem is corrected, they are generally very good offroad and efficiency-wise.

The mc2100/2150 is a good proven carb.
 
Yep. Start looking around for some numbers on it and see what you can cross reference on Google or with Edelbrock. Might even find the CFM rating stamped. I cant remember offhand as it's been several years since I ran an Eddy.
 
ok well im thinking if its a 390cfm im just going to get it rebuilt and tune it in and see how it runs... some people say they actually run good when tuned right and have the right power valve in them.
 
83CJLaredo said:
ok well im thinking if its a 390cfm im just going to get it rebuilt and tune it in and see how it runs... some people say they actually run good when tuned right and have the right power valve in them.
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Upgrading your ignition system is a good way to take advantage of aftermarket carbs also. A ford TFI is very simple to do and is a huge improvement over stock.
I ran a TFI ignition on my AMC 258 i6 / 4.2l , AMC 258 i6 / 4.2l /4.0 hybrid, and on my current 401 and love it.
 
83CJLaredo said:
Are there any write ups on the forums about it? And should i also be running a regulator for this carb because there isnt one on there, thanks
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A regulator? No, you shouldnt need one.
Im not sure if there is a TFI write-up on this site, it is however, all over the web. I should probably do a TFI writeup for this site...thanks for the idea :D
 
83CJLaredo said:
haha awesome well it will be much help thanks alot
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A short explanation:
The TFI upgrade for the AMC AMC 258 i6 / 4.2l is basically nothing more then the ignition system from a Ford 300cid engine.
The cap, rotor, plug wires, and coil are replaced. The advantage is the larger cap eliminates crossfire and offers better spark management, the Ford coil provides a hotter spark, the wires are 8mm vs. the stock 7mm. Plugs are re-gapped to .045.

The AMC V8 uses the ignition system from a Ford 460.
 
Hi - The clifford research website recommends the Weber carb, but they list them at $599 - Ouch. I noticed that the Clifford intake is a single plane intake, while the Offenhauser unit is a dual plane. The dual plane design is more consistent with off road torque requirements at lower rpm.

The Edelbrock carb site indicates the smallest is a 500cfm. This may be a bit much for the AMC 258 i6 / 4.2l . The problem is that the metering and vacuum sensing built into the carbs are designed to work within a certain flow range. The AMC 258 i6 / 4.2l may not flow enough air to make a 500 cfm carb work properly. I used to run a Carter AFB which looks similar to the Edelbrock design. It was great for street power, but I had a lot of trouble off-road with fuel sloshing and bogging when I got into rough terrain or off camber trails. The Carter had big float bowls and did not like getting banged around.

The Holley avenger design has a better float vent, and the spring loaded needle and seats perfrom well off road.


If Holley makes a 390cfm, that may be the right choice.

Good Luck
 
Also - I do run a pressure regulator on the fuel inlet line. I set it down to about 3 psi off road. I run 6-8 psi on the street. It reduces the pressure off the needle and seat when the jeep takes a bounce. I use a Napa part with an adjustable dial.
 
johnbilt said:
The AMC 258 i6 / 4.2l may not flow enough air to make a 500 cfm carb work properly.
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It wont. The head design is too restrictive as well as valve duration to short unless the cam is upgraded.

johnbilt said:
I used to run a Carter AFB which looks similar to the Edelbrock design. It was great for street power, but I had a lot of trouble off-road with fuel sloshing and bogging when I got into rough terrain or off camber trails.
Click to expand...
Carter makes Webers as well as Edelbrock carbs...and probably a bunch more brands. I've had 32/34's and Edy's as well stamped as "Carter" ;)






Good Luck[/QUOTE]
 
johnbilt said:
Also - I do run a pressure regulator on the fuel inlet line. I set it down to about 3 psi off road. I run 6-8 psi on the street. It reduces the pressure off the needle and seat when the jeep takes a bounce. I use a Napa part with an adjustable dial.
Click to expand...
A pressure regulator is recommended for the Weber also....at 3psi.
 
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