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Unread 03-29-2008, 04:09 PM   #16
sitdwnandhngon
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my 4.2 sucked when i got it, i did the nutter bypass, added a weber carb, and re timed it to 9* and it runs like a dream now, starts right up unless its super cold. Whenever something goes wrong i just reset the carb and adjust a few screws, sometimes change the float level and shes back running great again. No computer to mess with. I was looking at the 4.0 head swap to the 4.2 though, i heard there are great power and MPG gains. If you get a good deal on the 4.2 go for it and read up on the nutter bypass and get a good carb. It takes a little time but its cake once you got it down.

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Quote:
Originally Posted by northjeep View Post
4.2 has 0.2 more!
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Unread 03-29-2008, 04:42 PM   #17
northjeep
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4.2...all the way! (and back again).
4.2 can be fuel injected to produce gobs and gobs of power and torque.
4.2 can be cannibalized into a 4.0 block and you get a 4.6-4.7 stroker.
4.2 can take a 4.0 intake and gain horsepower.
4.2 can take a 4.0 ho head and intake to become very peppy indeed.
4.2 costs less as a crate replacement (just a little).
4.2 has 0.2 more!
4.2 parts are plentiful (so what).
4.2 (and many other engines) optimum average piston speed is between 1300 and 1500 feet per minute (ft/min) or 2000-2350 rpm cruse, which is easier on the engine internals than the equivalent piston speed on a 4.0....2300-2650 rpm. This is determined by the stroke.

At the high end of the 4.0 optimum piston speed with 4.88's and 37" tires, this makes a 72 mph cruse, compared to a more conservative 66 mph with the 4.2 and the same setup. This means less wind resistance for the 4.2 at 66 (above 55 you lose lots of fuel to the wind, in fact for every mile per hour over 55, wind resistance increases proportionately).
That means, if you want to slow down to a less aggressive speed (say 55mph) you will be outside the 4.0 optimum piston speed (by at least 200 ft/min). And never mind going up hill or hitting a headwind.
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Last edited by northjeep; 03-29-2008 at 05:23 PM..
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Unread 03-29-2008, 05:18 PM   #18
ElvisV04290
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Quote:
Originally Posted by northjeep View Post
4.2 has 0.2 more!
Hells yeah!
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Unread 04-10-2014, 02:07 PM   #19
rambo3489
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Revive! Wanted a little more discussion. Lets see if anyone has anything to add...
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Unread 04-10-2014, 02:25 PM   #20
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I just did efi on the 4.2L and am very pleased with it. It just hit 170k miles. Now I need to figure out why the tranny doesn't want to go into gear after 30+ miles of driving.
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Unread 04-10-2014, 02:50 PM   #21
89yjmainiac
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I just got the weber carb for my 4.2 I'm installing it tomorrow can't wait to have it up and running instead of sitting in the driveway especially with the weather supposedly going to be 73 in Maine Monday!
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Unread 04-10-2014, 02:51 PM   #22
zeus87gn
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The 4.2 is more awesome than the 4.0 because of the low rpm torque
The 4.2 is (IMO) a stroker motor - the length of the stroke is longer than the dia of the piston bore
Thanks to my axle gearing choice (4.10) and my tune, I start in 2nd gear on the street
I'll use first gear on a hill climb
5th gear could be taller for the highway, but oh well - 35s may be in my future
But it still runs great on the highway

Thanks to my Flowmaster 40, I have had people ask me if I have some strange V8 under the hood
When I tell them I have the old CJ motor and a carb, most are doumbfounded

It's a 50 something year old design, but it works well if cared for
A Jeep guru buddy of mine said once, "...they don't call it 'the workhorse' for nothing..."
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Holley Carb thread
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Unread 04-10-2014, 03:28 PM   #23
roadyrob
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Quote:
Originally Posted by roadyrob View Post
when I was looking for my heep, my main priority was a solid body and frame. i found an 87 that had been a motor home tow behind, completly stock, with a poorly running 4.2. swapped out the carb and ignition for a weber and hei. now it runs great, gets about the same gas milage as the 4.0 ( 14-15 around town, 18 highway)
its easy to work on and has plenty of power. the HP and torque are at low rpm's where you can use it. great motor if they are set up right.
boy is this an old thread, still think the 4.2 is a great motor but a lot has changed, the motor has been rebuilt with a hotter cam and head work. still running the same HEI from CRT and haven't had to touch it. the weber has been replaced with with a offenhauser manifold and holley 390 cfm 4 bbl.
should have went with it to begin with. finally opened up the exhaust with a 2 1/2" high flow cat and muffler.
go with a 4.2 if you like to tinker and modify, amc left a lot of easy HP.
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Unread 04-10-2014, 03:41 PM   #24
Old4X
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Originally Posted by igofshn View Post
Serious carb issues and less HP than a 2.5 4 cyl TJ. Don't waste your money. Post 91 is what you want.

LOL! HP is not what turns the tires, Torque does, and torque at low RPMs is what the 4.2 is better at than the other engines.

If you aren't an expert on carburetors, get a 4.0. Do not even look at the 4 poppers, you WILL be sorry.
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Unread 04-10-2014, 03:45 PM   #25
rambo3489
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Quote:
Originally Posted by roadyrob View Post
boy is this an old thread, still think the 4.2 is a great motor but a lot has changed, the motor has been rebuilt with a hotter cam and head work. still running the same HEI from CRT and haven't had to touch it. the weber has been replaced with with a offenhauser manifold and holley 390 cfm 4 bbl.
should have went with it to begin with. finally opened up the exhaust with a 2 1/2" high flow cat and muffler.
go with a 4.2 if you like to tinker and modify, amc left a lot of easy HP.
Your running a 390 CFM? Isn't that a little much for the 4.2? Does the Offy allow for the bigger carb? I'm running a 2150 stock intake and a header.
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Unread 04-10-2014, 04:17 PM   #26
roadyrob
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Quote:
Originally Posted by rambo3489 View Post
Your running a 390 CFM? Isn't that a little much for the 4.2? Does the Offy allow for the bigger carb? I'm running a 2150 stock intake and a header.
the 390 holley with vacuum secondaries works good, i'm sure the offenhauser manifold helps along with cleaning up the casting ridges in the head ports and port matching.
a few people here run hollley 4bbls, superj runs a 450 cfm on his stroker and wendell runs a 470 truck avenger on his modified 4.2, I'm sure there are a few more.
i thought I'd have to mess with the vacuum secondary springs and power valve but it works good as it is.
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Unread 04-10-2014, 06:04 PM   #27
Tom95YJ
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I like my 4.2l I can tell you with stock gearing on 33's it is way more drivable then the 2.5L TJ and 2.5L YJ that I had with 33's on stock gearing before I regeared them both.

As far as carb issues, The carter sucks no way around that. I picked up a Weber 38 carb, and CRT Hei distributor they should really wake up the 4.2L, In the future I plan on a 4.0 Head swap and a cam swap that should get me into the 210-220hp 260-280tq range if I cam correct
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Unread 04-10-2014, 06:52 PM   #28
superj
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I like mine, no problems on my 450 holley four barrel and offenhauser intake. Its a solid motor and tows easily ( I tow more then off road, no 4x4 places around here ). I am actually going to put my motor and trans (tf999) into another jeep when I do my 302 swap this summer because the motor is so solid.


The 4.0 is solid also though. Had a ton of those also with over 200k when we sold them.
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Unread 04-10-2014, 06:57 PM   #29
jaycal3
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I had nothing but issues when I had my 4.2... Switched to a Weber from the Carter and it still sucked. I couldn't get the Jeep to pass emissions here in Ct no matter what I did so I couldn't get it registered. Never ran smooth.

I love my 4.0... and its fuel injection
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Unread 04-10-2014, 08:01 PM   #30
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Sometimes it works out that way. Same with any engines though, fuel injected ones too
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