84 CJ7 refuses to idle - JeepForum.com
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post #1 of 76 Old 04-06-2020, 11:48 AM Thread Starter
BeefStewer
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84 CJ7 refuses to idle

Hey all,

This is my first venture into the forum (and mechanics in general), so please try and take it easy on me.

I have a 1984 CJ7 with the AMC 2.5L 4 cylinder. Carter YFA feedback carb.

I have been pulling my hair out trying to get this thing to idle consistently.

First off, there is no external mixture screw, not a place for one that I can find.

I seem to remember reading somewhere that the mixture is controlled through setting the metering rod, since the mix solenoid on the side does the rest.

I want to disable the solenoid, and I've read a write-up on how to do the nutter bypass the 4 cylinder engine so I am not opposed to anything being removed. However, I don't have the means to replace the Carter carb, not do I want to, for whatever reason, unless the thing is worn out.

I have already rebuilt the carb and replaced all fuel related components except for the hard lines on the frame.

I also have replaced the ignition box, coil, dist cap and rotor, plugs and wires.

Everytime I try to get this thing dialed in, it will work fine for a bit, then when I try to start it up a little later, it won't work.

Just earlier today, I set the metering rod so that it would actually run, and it idled for about 5 minutes before I shut it off. It was really rough during this time with the rpms jumping around, so I looked at the plugs to see what my mixture looked like. It seemed kind of lean, but I had a vacuum gauge hooked up during this as well and it read about 18 inches at 1000rpm.

Anytime the rpms went lower, the vacuum level tanked, and the Jeep would run way rougher.

During this time, it was about half choke. I let it run, and it seemed to run worse at any other level of choke.

I stopped it, and when I started it back up, the electric choke had opened up farther (about 3/4 choke) and now all it does is cough and backfire.

I know it's not a common engine, but I also know it's very similar to the 258; so maybe someone knows something that could help?

Sorry for the long post, but I figured more information = better.

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post #2 of 76 Old 04-06-2020, 01:07 PM
Matt1981CJ7
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First, welcome to the forum. You came to the right place to get help on your CJ.

It sounds like you have a major vacuum leak. As the choke opens, the mixture leans out. So, with a vacuum leak, the engine may run OK when the choke is closed, but will run rougher as the choke opens.

Your '84 engine had a bunch of troublesome vacuum controlled systems from the factory, mostly for emissions purposes. If you don't have to worry about emissions testing, I highly recommend the Nutter Bypass to eliminate about 90% of the vacuum leak possibilities.

Keep us posted,

Matt


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post #3 of 76 Old 04-06-2020, 01:44 PM Thread Starter
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Matt,

Thanks so much for the reply. I'm very new to this, so it's really nice to have a community that is so willing to help.

I've got the wires for the nutter bypass ran and hooked in already.

I haven't been able to set the timing yet just because when it does idle, it's so rough that I can't get a good reading.

I have every vacuum port plugged save for PCV, choke pulloff and bowl vent (is that even vacuum?) and all of those are new hoses.

I messed with the metering rod some, and I've got it to the point where it'll idle in a range of only +-100 rpm with the choke wide open.

What confuses me though is that my vacuum gauge reads only 10 inches of vacuum at 1000rpm. Could this be timing related? There's a chance that my mix is still slightly rich, but this is the first time it's idled halfway decent so I'm a little hesitant to change that again.
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post #4 of 76 Old 04-06-2020, 03:02 PM
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10 of vacuum is way low. Im guessing you still have a vacuum leak somewhere. If you have a small propane torch, turn the gas on (dont light it), then work the torch all around your carb and intake manifold area. If the idle changes at any spot, youve found your leak. A smoke test works well, too, but it requires more specialized equipment.

Also, post some good pics of your engine compartment from various angles. Perhaps we can spot something youve overlooked.

Have you removed all the disabled emissions crap after doing the Nutter?

Matt
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post #5 of 76 Old 04-06-2020, 03:48 PM Thread Starter
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Unfortunately I have not physically removed anything yet.

All the existing lines are still in place, just unplugged/capped off.

I just adjusted the timing after getting it to idle slightly more consistently and it pulls a few more inches of vacuum now, but still low (12-13")

Engine seems to miss a little below 1500 rpm, which I attribute to a vac leak like you said.

I did find a year in one of the carb gaskets (between air horn and float bowl) that I will need to order a kit to replace. I gain a good 200 rpm when spraying carb cleaner at that area, so I would assume it's a decent size leak, but I would imagine not all of it.

Gasket between carb and intake manifold probably needs changed as well. Can't gain any rpm when spraying carb cleaner, but I can see it starting to separate.

Intake/exhaust manifolds gasket (is this one piece?) looks suspect, but it's hard to tell since this thing had the same valve cover leaking issue as the 258s. So there's a few years worth of oil still built up on the manifolds. New gasket though, so no new leaks for now.

I have to keep reminding myself that this Jeep sat for around 30 years, so there's probably plenty of things I will need to do yet.

Btw, how would I post pictures? I've got the ones you mentioned, but I have no idea how to post them.
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post #6 of 76 Old 04-06-2020, 04:35 PM Thread Starter
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I suppose I forgot to mention, the engine does pull about 15 inches of vacuum at 1500ish when idling down at startup.

It does lower as it idled down, and holds when it gets to 1000 (recommended idle speed).

I'm not really sure why I get more vac at a higher rpm, because if I give it slightly more throttle, vac goes down, like I'd expect, and I thought that the lower it went the more vac there was.

I'd try and get the idle lower, but it doesn't seem to like it, gets too rough/starts shaking pretty bad. Not a big deal, but a guys gotta try right?

It runs really great at 1500 and above, even though I pull a good vac at 1500. I'm not the most knowledgeable about this, so don't take this the wrong way (just trying to understand this better), but if I have a vac leak, why does it pull a better vac at a higher rpm?
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post #7 of 76 Old 04-06-2020, 05:40 PM
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Welcome to the forum and the Jeep World (Just Empty Every Pocket to pay the Jeep-gods).

Your vacuum is higher at higher RPMs because most likely you are using ported vacuum. Manifold vacuum will drop as engine speed increases, but ported vacuum will increase. 10-15 inches of manifold vacuum is excessive low! It sounds like you have ported to me.

Take good, clear, and precise pictures so other may see what you are dealing with, what you are currently doing, so other may provide better guidance to you.

You can do this. It will get frustration, I promise, so be patient. You will learn so much from this process. What you fix now can help yourself, or others, in the next issue that comes. Yes, there will be more issues. Welcome to Jeeps and Jeeping.

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post #8 of 76 Old 04-06-2020, 06:22 PM Thread Starter
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I will post pictures as soon as I figure out how.

I have my vacuum gauge hooked to the lowest vac port I can find, and it is a port attached to the manifold.

As I increase the rpms, the vacuum does drop, but the highest value I get is at 1500 rpm. Anything above that drops with acceleration, and anything below it drops to about 12-14" (can't get a really smooth idle). It does seem like it wants to be more, but the rpm can't smooth out because it seems to misfire at lower rpms.

Is it possible that I get less vacuum because this is a smaller engine? The vac gauge that I have says 14-18 inches is good. (Though it is from the good old Harbor Freight so who knows.)

It still seems like the mix is wrong, so maybe that as well?

I actually finished the Nutter on it earlier, and it seemed to improve slightly (ran smoother) but not a ton.

I had to set the timing WAY advanced. Not actually sure exactly what the value is (Jeep is stuck outside, too bright to see light well) but if I had to guess, I'm close to 20-24* BTDC. Does this sound right? The guy that wrote up the version of the nutter for the 150 had his at 18* BTDC. I don't quite fully understand the whole timing concept the best, so is this dangerous?
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post #9 of 76 Old 04-06-2020, 10:01 PM
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Your high range of vacuum is the low end of good.

I don't know much about the 2.5L and they're particular issues, but all engines require three things to work: Fuel.Air.Spark. The nuances are the different ratios, timing, yada yada.
With the nutter you are dealing with spark and the control of spark. That is not your issue.
You have a vacuum leak, which is an air problem. Too much air. This extra air leans the fuel/air mix ratio so the computer is trying to regulate the amount of fuel through the metering rod in the carburetor from information received via various sensors installed throughout the engine. Bypassing the computer (nutter bypass) means the regulation is sort of fixed in a sense.
Find the leak is paramount.

You stated you found a tear in a gasket and registered a change in idle response. That sounds like a good location to fix. Keep at it.

O\IIII/O
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post #10 of 76 Old 04-06-2020, 10:08 PM
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May I suggest that you fill out your profile so other can see what you a driving. It waste everyones time asking "what engine, transmission, gear ratio, tire size...", whatever the question you may have.
You asking for help, you need to give the background information up front. It saves time. Help us help you. I know you have a lot you're dealing with and it all seems over your head. Next time you take a break, fill out you profile. It will help you and the community.
Good luck.
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post #11 of 76 Old 04-07-2020, 06:59 AM
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OP, you'v e been given some slightly wrong information. Manifold vacuum should not decrease with higher RPM, when you are just revving the engine in the driveway. It should remain high (around 18" depending on your elevation) at all RPMs when there is no load on the engine. Manifold vacuum does decrease when driving under load, but it should remain high and fairly steady when revving in the driveway.

So back to your problem. Any vacuum leak that causes a noticeable change in RPM when sprayed with carb cleaner is a fairly sizable leak, and it is most likely the cause of your low RPM performance, or at least contributing to it.

If you are using the forum from a computer, just drag your pics from your computer to the area below the reply box labeled "Drag and Drop File Upload". I'm not sure, but you may have to have some minimum number of posts before the forum allows you to post pics. Perhaps someone else can confirm that.

Matt


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post #12 of 76 Old 04-07-2020, 07:20 AM
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I love that 2.5 AMC with the YFA. It was a simple motor with a feedback carb that as a heckuva less complicated than the Carter BBD.

As Matt says, you have a vacuum leak.

OR, the mixture solenoid might not be working. I *think* it still works at idle. Let me dig out my old Cherokee Chilton and take a look at the circuit.

If the solenoid diaphragm ruptures, it'll go way rich.

The Sol-Vac solenoid controls the idle speed. There should be vacuum going to the Sol-Vac, as well as an electrical solenoid.

The vacuum diaphragm in that Sol-Vac will blow, and create a vacuum leak. In trying to maintain idle, the electrical portion of the Sol-Vac makes the idle surge.

Is the Sol-Vac still there? If so, throw suction on it and see if it operates. If not, that's your vacuum leak.

Thoroughly check it, and I'll grab my Chilton from back then and see if I can't use it to remember back to 1995

Josh
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post #13 of 76 Old 04-07-2020, 08:01 AM
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Here's the system you should have.

Change that O2 sensor, first off -- those older ones don't last. If I recall, they'd get replaced every 50,000 miles. I doubt most have been replaced.

Then, go through those hoses and replace them all. They're going to be cracked with age.

I was seeing issues with this stuff in vocational school in '94 and '95, stuff that was only 10-15 years old. All the Jeeps came to me; J trucks, Cherokees, the teacher's Grand Wagoneer, CJs. I saw more issues with that Sol-Vac and cracked hoses than with anything else.

Josh
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CEC System 4-150.jpg  
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post #14 of 76 Old 04-07-2020, 08:41 AM
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It would help if all the files would attach... just noticed they didn't.
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post #15 of 76 Old 04-07-2020, 09:21 AM Thread Starter
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Thanks guys,

It does seem like I need a minimum number of posts to post pictures.

I have ordered a new carb kit online, as well as a new gas cap since mine doesn't seem to be venting correctly.

As for the mix solenoid, i have it unplugged, and the internal passage for it blocked off. Otherwise it was sending everything full rich.

I also have the Sol-Vac unhooked from vacuum since I do believe that it is another leak source. I still have the electric portion plugged in though, but that is mostly because its the same plug as the electric choke, and that does still work.

I have removed the idle screw that contacts the Sol-Vac from the idle linkage so as not to interfere with the idle, and i have replaced the other idle screw (not the fast idle one) with a longer screw to compensate. I have been using this as my curb idle adjustment now.

Now, it's very likely I misunderstood something, but wouldn't the Nutter Bypass basically disable any functions the O2 sensor, mix solenoid, etc. control? I thought that it removed the computer from having any control over the main functions of the engine. Please let me know if I misunderstood this.

Also, I filled out my Jeep info in my profile, there's some info I haven't filled out yet like gear ratios, but I will get those when I figure them out. Some of that stuff may change, since this whole thing needs rebuilt.
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