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Old 09-11-2019, 08:41 PM
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Question Stumbling at Idle | 1990 Jeep YJ Carbureted

I did the HEI conversion to my jeep a couple weeks ago and while it runs good, the idle is off a little. I think it might be something to do with timing, but I am not used to working on carbureted vehicles so I am not the best at it. So here is what I have attempted, and it has made it better, but not stopped it.


1. Turned idle up to Approx. 800rpms, I'll turn it down this is just for timing.
2. Attached vacuum gauge to manifold vacuum, by brake booster
3. From there I messed with the mixture screws until I got the highest possible vacuum I could
4. I set my timing to 8* BTDC, vacuum advance is disconnected. That is where it seemed to run best in the 6-8 range. Should I advance more?

(I also installed new copper sparkplugs and set the gap to 0.045. I know stock is 0.035, but I read on another forum post to set the gap between 0.045-0.050 on an HEI setup)

I have not tried connecting the vacuum advance since I did this, but before hand it would always make it backfire or lug. It was a fine line between backfiring and lugging, less that 1/4 of a turn on the adjustment screw for the vacuum advance.

Well that's all relevant information I can think of at the moment. If there is anything else you need to know please ask.

Does anyone have any advice for me on how to get the idle to smooth out completely? I know it has to be something with the new carburetor or HEI distributor setup, it did not do this prior to the conversion.


I tried to get some video of it at idle, but it sounds smooth on video of course. Here it is anyway.

https://drive.google.com/open?id=10O...MGYVgoKSHc4MXW

https://drive.google.com/open?id=10J...ZP8LdpkH6heEyV

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Old 09-11-2019, 09:05 PM   #2
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I have the timing on mine set at 10*, with the Vac advance connected. I would ask about your timing chain being a little stretched, by my idle is a little rougher than my '13 honda but I give that to the 25 yr difference.

How is it on acceleration from a stop? That seems to be a common stumble when cold that can be corrected most times with the choke preload.

Copper core plugs are good, the cheaper the better, I like the autolites, gapped to .45-.50 is good.

I'd do a double check on your remaining vac hoses to make sure there are no small cracks etc.

One last thing to remember is that the carb is not dialed as tight as EFI, There's gonna be a gap between Idle mix and when you start to open throttle, You might try to tweak it with the idle a little lower, like maybe 600-650Rpm's

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Old 09-11-2019, 10:02 PM
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Acceleration is smooth from a stop. The stumble either goes away, or is unnoticeable passed about 1200rpm.

Autolites are the sparkplugs I have so glad to hear they are good ones.

I did go over vacuum lines, but it never hurts to look again, I removed the CAD system from the front axle and I don't remember capping the vacuum line for that. I'll give it a look.

I will also adjust the carburetor at ~650 rpm and see how that goes.

Thanks for the info.
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Old 09-11-2019, 10:44 PM   #4
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Quote:
Originally Posted by brrang View Post

I did go over vacuum lines, but it never hurts to look again, I removed the CAD system from the front axle and I don't remember capping the vacuum line for that. I'll give it a look.
Your Cad vacuum typically comes from the last vac port at the back of the manifold line,Tee'd off the line that runs into the firewall to operate the defroster gates in your heater box. You can cap it there.

Unrelated... you may wish to rotate your fuel filter so that the return line to the tank is at 12 oclock in relation to the carb supply at 6 oclock, so that you get the carb filled first then fuel is returned to tank, and maybe add a one way restrictor inline if you don't already have one.
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FSM Manuals https://www.dropbox.com/sh/0vfmk6jc5...v7lceZk7a?dl=0
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Old 09-11-2019, 10:48 PM   #5
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The vacuum advance hose to the distributor should be connected to manifold vacuum rather than ported vacuum.
Set the timing with the vacuum advance hose disconnected from the distributor. Plug the hose or pinch it off. Reconnect the hose prior to test driving the Jeep.

I have a HEI and a non-stepper Carter BBD carb. I'm running the copper core Champion plugs gapped at .045. Mine idles best at around 750 RPM.
The main difference is that I have my timing set at 12*. You might try advancing your timing to see what happens.

Bear in mind these are old vehicles and most of them come with an unknown history of maintenance and use or abuse. Unlike modern vehicles, (or these vehicles when they were new) each one will require a slightly different setting for things like timing, carb adjustment and plug gap.

Let us know what cleans up your idle.
Check out the info in this link.

Easy Fix for Jeep 258 Carter BBD Idle Problem

Good Luck, L.M.
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Old 09-12-2019, 09:40 AM   #6
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If you're going to be setting the timing by "feel" or using a vacuum gauge to get highest steady vacuum then you NEED to use a "real world" situation with the vac adv hooked up.
If your going to follow the convoluted instructions to get a factory spec timing set than good luck, It's alot more than unhooking the advance and checking timing marks.
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Old 09-12-2019, 10:42 AM
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Quote:
Originally Posted by Luckymac View Post
The vacuum advance hose to the distributor should be connected to manifold vacuum rather than ported vacuum.
Set the timing with the vacuum advance hose disconnected from the distributor.
It goes to manifold vacuum? I thought it should go to ported vacuum so that the vacuum increases with RPM rather than decreases. I am gonna work on it today so ill try everything you guys have told me about.

Thanks a lot.
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Old 09-12-2019, 11:00 AM   #8
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Quote:
Originally Posted by brrang View Post
It goes to manifold vacuum? I thought it should go to ported vacuum so that the vacuum increases with RPM rather than decreases. I am gonna work on it today so ill try everything you guys have told me about.

Thanks a lot.
I thought the same thing. I'm a bodyman, not a mechanic. There is a few posts on here about why the vacuum needs to be taken from the manifold.
Hopefully the poster(s) that have that information will post it again here.

I googled "vacuum ported vs manifold" and found this. It's about Chevy engines, but the mechanics are the same.

https://www.chevellestuff.net/tech/a...r_manifold.htm

There were a bajillion hits. Too much to read, but you'll get the idea.

Good Luck, L.M.
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1987 YJ-4.2L-Standard Shift- Re-manufactured Carter Carb-2" body Lift-31X10.5X15 BFG KOs-190K Miles No back seat.
HEI distributor with computer and all related relays and wiring removed.
12K Badlands winch with dual batteries.
Warn front bumper. Matching imitation Warn bumper on the rear.
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Old 09-12-2019, 02:26 PM
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Thanks for all the help and advice. I spent about 2 hours messing around with the timing and carburetor and it seems like no matter what I do it doesn't change. The only changes I could get out of it was to make it worse when messing with the carb adjustments.

I did find a small vacuum leak coming from a small brass fitting behind the choke mech. (see picture) so I plugged it and that might have smoothed it out a very little bit.

When I attempted to hook the distributor to manifold pressure it would kill the motor almost instantly. I adjusted the screw in the distributor from lock to lock and it made no difference, would just kill the motor.

I am going to spend some time reading and watching videos on carburetor adjustments and timing. I simply don't know enough about this old school stuff to feel confident in the changes I am making. The jeep is older than I am, after all.

I also took a video of it idling from the exhaust because you can hear the rough idle much better. Maybe that will help some of you guys find something I am missing.

https://drive.google.com/file/d/10RZ...ew?usp=sharing
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Old 09-12-2019, 04:22 PM   #10
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Double check that you have the correct firing order.
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Old 09-12-2019, 09:12 PM
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The firing order I have set up is 1-5-3-6-2-4 in a clockwise rotation on my distributor.
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Old 09-13-2019, 09:05 PM   #12
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Old 09-14-2019, 04:27 AM   #13
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Make sure you don’t have a leak from the intake manifold, I speak from experience
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Old 09-14-2019, 01:56 PM
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Make sure you donít have a leak from the intake manifold, I speak from experience

I just replaced the intake and exhaust manifold gaskets... twice because it still leaked the second time. I have triple checked it and it definitely does not leak now.

I am starting to think it might be a valve or something sticking causing an inconsistent back firing. I took it for about a 30 min drive and it did backfire on me twice, sounded like it was out the exhaust.(sounded like a big puff of air, not a loud pop) I might run some cleaner through it and compression test it.


On a side note does anyone know why an oil pressure gauge would stay pegged. I assume it is in my wiring, because I replaced my sending unit and it had no affect. I searched all the forums and they all say to replace the sending unit, that's why I started there.

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