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Author Topic: 4.2 CJ7 "surging" at 60 mph under load.....  (Read 1389 times)

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johnnyboy

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4.2 CJ7 "surging" at 60 mph under load.....
« on: October 05, 2021, 12:41:42 PM »

Engine pulls well in normal use, but under sustained load it loses power for a few seconds then picks up then loses power again etc.
Maybe 2 seconds of power loss then 2 seconds of power then 2 seconds of power loss....
Ease off and it is fine. Its like having a speed limiter.Not a problem in normal driving (it's too noisy to go much above 60 anyway) but off-road up a hill..... or even on a motorway up a hill....

It sounds to me like not enough fuel getting through to the carb under sustained throttle, but the fuel pump and filter are quite new (just a few hundred miles...).
 Standard Carter 2bbl, rebuilt a couple of thousand miles ago.
On general principles I gave it a new coil, distributor cap and leads (previous were possibly original....) and maybe the engine ran a bit better, but no change to the surging.
If it had a cruise control I would blame that....

Any suggestions welcome?
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frosty 1

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #1 on: October 06, 2021, 04:56:05 PM »

still sounds like a fuel supply problem, is there a filter in the carb where the pipe comes in.
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johnnyboy

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #2 on: October 06, 2021, 09:15:50 PM »

still sounds like a fuel supply problem, is there a filter in the carb where the pipe comes in.

Hmmmmmmm.
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gazjeep

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #3 on: October 07, 2021, 07:27:04 PM »

Bit out of left field but does your carb have a vacuum pipe to the engine ?

I know mine on an Essex V6 had a small hole which affected its running a little, not like your surging symptoms though, just throwing it out there as something to cross off the list of possible' s 
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Dave The Sparky

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #4 on: October 08, 2021, 07:10:41 AM »

Another one for you.
I had a kinked fuel hose on top of the tank that made my Jeep do this, allowed enough fuel through to run normally 99% of the time but when you needed the extra power going up hill it just couldn't pull enough fuel through and stumbled, back off the throttle and it was fine again.
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Brynjaminjones

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #5 on: October 18, 2021, 01:42:42 PM »

Is the fuel filter on a 4.2 one of those with an extra return nipple? If so, I've heard of a few people having strange problems when those are oriented in the wrong direction. They're meant to rotated to have the return line right at the top.
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Dave69

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #6 on: October 23, 2021, 01:10:05 PM »



Had the same with mine running the cater 2bbl carb, the carb is not designed for the 4.2 engine, never solved the issue and the mpg just bottomed out. I swapped to a webber and no issues at all. Setup of the webber was simple as its based on the idle, then just swapped the main jets for what worked and never looked back.
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johnnyboy

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #7 on: October 25, 2021, 02:44:04 PM »



Had the same with mine running the cater 2bbl carb, the carb is not designed for the 4.2 engine, never solved the issue and the mpg just bottomed out. I swapped to a webber and no issues at all. Setup of the webber was simple as its based on the idle, then just swapped the main jets for what worked and never looked back.

Which Webber did you use? Is it a straight fit on the manifold? How about the throttle linkage - does the cable just fit on, or do you need to modify? What about Choke? Any suggestion for a supplier?

- Its getting worse, and over the weekend I have checked all of the possibilities that people have raised below (thanks for the ideas though guys!)
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Dave69

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #8 on: October 27, 2021, 02:45:48 PM »


I bought a 38 DGS off fleabay but i also got at throttle linkage as well, but only used some of the parts as it was only a case of connecting the cable. Nothing relay technical there.

Mine didn't have a choke, so a few pumps of the throttle and the engine fires, there are options for electronic, manual and thermostat but these are identified by additional letters in addition to the DGS identifier.

if your manifold is a single BBL (two bolt) then you will need an adapter stand-off for the weber to fit as its a dual barrel carb. If your a 2-BBL then you may only require an adapter plate to suit the bolt centers.

To be honest it was the quickest swap for a carb i have done, the time was spent working out carb orientation to suit the throttle cable and fuel supply. I also went for an electric fuel pump and a regulator, mainly to know there is fuel supplied tot he carb as opposed to waiting for the mech pump to prime the system (personal choice).

Air filter i used is just a standard rectangle filter.

Plenty of information on tinternet from stateside to confuse the situation, just some links below

https://www.carburetion.com/Products/ProductDetails.aspx?Part=99004.551    (adaptor plate link)

https://www.jeepforum.com/forum/f8/weber-set-up-tune-discussion-1051137/
http://www.redlineweber.com/html/Tech/carburetor_set_up_and_lean_best_.htm
http://www.redlineweber.com/html/Tech/38_dgas_tunning.htm
https://www.quadratec.com/sites/default/files/installation_instructions/K550%20K550-38%20NEW%20LABEL%20%281%29.pdf
https://www.quadratec.com/Assets/Installation/4412.pdf



If you persue the route, alot of people say they are a nightmare to setup, this is because of over thinking the process. Get the idle set first and its correct so engine is running on idle circuit and not pulling fuel through the main jets. Once that is done its a case of drive and test to see how the engine pulls. This may require swapping for a larger jet or smaller (depending on what the carb was used for if 2nd hand). You can get a rough idea from a bit of google research from other people to for a starting point to save on buying numerous jet sizes.

I cant remember what im running without taking out the jet and looking but the engine idles smooth and pulls through the gears no problems of lagging as seen with the old 1BBL carb. Fuel MPG to be fair is not considered on these






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Dave The Sparky

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #9 on: October 28, 2021, 06:48:50 PM »

Just another thought, you haven't been near the new E10 fuel with it have you???.

If you do decide to change away from the Carter for a Weber carb i would go here they are very helpful https://www.webcon.co.uk/ they are down in Middlesex and will sell you a genuine Weber, the US versions are Chinese copies and not very good.
You would still need an adapter plate making for it though which would be enough to make me try and get the Carter going properly first.
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wildwood

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #10 on: October 29, 2021, 09:24:28 AM »

Hello and yes I’m still lurking…..

I also believe that there are two choices of Weber carb….
One has the smaller primary and bigger secondary choke/ barrel
The other has two large matching choke/barrels.

I guess big jugs make more power…

Might use more fuel though :icon_super:

This information….came from the filing cabinet that is my head and as such is at least 5 years or more old…
Happy Friday Dastards.
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Dave69

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #11 on: October 29, 2021, 02:21:03 PM »



there is a 36/38 dgv where you run on the smaller choke at low speed and when the foot gets floored the 38 choke gets used, the difference in mpg between this and the 38/38 is negligible hence i went for the dual 38. my current mpg is 10 to the gallon !!! i think going for the smaller will only add a couple of mpg.


avoid the Chinese copys like the plague as they are a very good copy.

E10 shouldn't be an issue given how basic the fuel delivery system is or you have an issue with the mechanical pump not supplying
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Dave The Sparky

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #12 on: October 29, 2021, 07:49:41 PM »

E10 shouldn't be an issue given how basic the fuel delivery system is or you have an issue with the mechanical pump not supplying

That isn't what i meant i was thinking more of the E10 causing deterioration of the rubber and plastic parts of the fuel system, maybe thats a future issue?.
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johnnyboy

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #13 on: November 02, 2021, 01:34:50 PM »



i went for the dual 38. my current mpg is 10 to the gallon !!! i think going for the smaller will only add a couple of mpg.


Ten!!!! I am used to getting 22 to the gallon on highway, 19 round town on the Carter!

- though it has gone down to 16/17 this last few weeks with the surging going on...

I think I might persevere with the Carter for a bit if I can!
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johnnyboy

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #14 on: November 02, 2021, 01:43:11 PM »

E10 shouldn't be an issue given how basic the fuel delivery system is or you have an issue with the mechanical pump not supplying

That isn't what i meant i was thinking more of the E10 causing deterioration of the rubber and plastic parts of the fuel system, maybe thats a future issue?.


I have replaced all the pipes and the carb gaskets are new, so they should all be "E10 OK".

I also have a new sender gasket kit on order on the same assumption.

I wouldn't have thought a CJ would have been sophisticated enough to worry about the "flame front propogation" issues that seem to be affecting a lot of higher performance 2000 - 2010 cars (though I have tried a tankful of super in it at the moment "just in case" - that does not seem to be making any difference...)
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Dave69

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #15 on: November 07, 2021, 11:25:20 AM »



the BBL carb was intended for use on the 2.5ltr engine but was installed on the 4.2 with just a jet change to suit so woefully undersized from the point go. so any other larger size carb will be an improvement. Plenty of info to be fair on tinternet from the states with good and bad reviews for Webber and Motorcraft carb swaps, most of these derive from people not understanding how to set a carb correctly.

Looks like you have covered the simple points for possible causes and whats left is the critical items like carb. My O.E carb got to the point of returning 10mpg due to poor running so i swapped to the webber which gave 12mpg and a feeling of power and smooth delivery. Unfortunately the power from the engine is not great

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wildwood

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Re: 4.2 CJ7 "surging" at 60 mph under load.....
« Reply #16 on: November 07, 2021, 11:28:30 AM »

And that’s why our US friends grab a cheap LS 5.3 and remote injection kits…..

Tuneable and relatively frugal…..

Sadly not so easy to do from Blighty…. :icon_super:
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Dave69

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