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If I press down more than 1/4 throttle engine stumbles and shuts off

Tiny
JDUB80
  • MEMBER
  • 1986 JEEP CJ7
  • 4.2L
  • 6 CYL
  • 4WD
  • 106,000 MILES
I've seen/read numerous posts on here about CJ's stumbling, tried most suggestions but can't seem too get mine running right.

Vehicle: 258, with Holley truck avenger 470 carburetor. Nutter bypass. Computer is sitting in the back seat as of an hour ago. (All wires cut by PO, so I pulled it out of the dash today.)

Carburetor details: 37 squirter, currently 10.5 power valve (initially had a 6.5 then tried a 2.5 trying to get something to change to no avail.) Can't remember primary/secondary jets but I verified that they are stock. Full manifold vacuum with 18-20in, approx 8* initial advance. (Need to check again. I don't trust my timing light. Think I broke it)

Symptoms. Well really only one starts and runs great, as long as I only use the first 1/4 or so of the throttle. Anything beyond that, at any time, the engine stumbles and will quit running.


I'm sure I'm leaving it details that would help, but I'm not sure what they are yet. Let me know and I'll definitely find out or provide them as soon as I can. I am brand new here, so I don't know how the notifications work yet, but I'll be checking in anyway.

Thanks in advance!

Sorry, currently no engine pics. I'll add some tomorrow though. Advice on what to photograph is welcome.
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Friday, July 5th, 2019 AT 1:31 PM

77 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,572 POSTS
Welcome to 2CarPros. Off hand it sounds like it could be timing related. Does it have the computer controlled distributor or one for a carburetor? Does it shut off setting still or only if you're in gear?
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Friday, July 5th, 2019 AT 3:28 PM
Tiny
JDUB80
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It used to be computer controlled carburetor and timing, with vacuum advance. However all of that (carburetor, excess wiring, electronic carburetor controls, extra vacuum ports - still has vacuum advance at distributor) has been removed or bypassed (nutter bypass). PO cut the wiring harness and I pulled it and the computer out of the dash today.
I probably should've said this is a 5 speed, I'm just not sure which transmission it is yet

It sat and idled over an hour a few days ago. Only dies if I give it more than 1/4 throttle and don't let off. Otherwise I can drive it, with no power or speed. Highest speed I've been able to go I'd 56 mph, downhill.

Also, I noticed that I misspoke in my original post. Timing is at at 8*btdc (I said 8* advance)
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Friday, July 5th, 2019 AT 5:28 PM
Tiny
STEVE W.
  • MECHANIC
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Okay, I would first look at the distributor and be sure it has the vacuum advance connected to ported vacuum. Next connect a timing light and watch to see if the timing actually changes with engine rpm/vacuum change. If it is not changing the advance diaphragm could be bad.
Did it run okay and then this started happening or was it like this when you first got it?
Next would be to determine if it's running rich or lean. If it will shut off just from the throttle then see if putting your hand over the intake to cut the air down changes it's behavior. If it suddenly runs better at 1/4 throttle it's running lean. I'm thinking it won't and is actually too rich as it comes off the idle circuit.
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Saturday, July 6th, 2019 AT 7:24 AM
Tiny
JDUB80
  • MEMBER
  • 52 POSTS
I'm just waking up, but here's a partial answer. More, plus pictures shortly.

It's connected to manifold vacuum now due to several posts I've seen here saying to use it.
Vacuum advance is operational. I've even applied suction the vacuum line to watch it move
If I hold the choke plate nearly closed with my hand it won't shut off and seems to run great. Made me think it was extremely lean, so I increased squirter from 35 to 37.

With that said, I just remembered I didn't change the idle adjust gaskets when I cleaned the bowls and metering blocks. Could that be my problem? (Air coming in there.)

Anyway, I'm going to grab some breakfast and coffee then head outside for pictures and such.

I haven't said it yet but thank you for your help!
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Saturday, July 6th, 2019 AT 8:52 AM
Tiny
JDUB80
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A few pictures under the hood. Seems like I read I'll only be able to upload three at a time. So here goes nothing.
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Saturday, July 6th, 2019 AT 10:34 AM
Tiny
JDUB80
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Round 2
2 this time, 2 next.
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Saturday, July 6th, 2019 AT 10:36 AM
Tiny
JDUB80
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That's all I got for now. Going to check timing with a borrowed light that I think is better than mine.
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Saturday, July 6th, 2019 AT 10:39 AM
Tiny
JDUB80
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Here's a new development. Using my father in laws timing light I can't see the mark on the balance.
Def starting to look like a timing issue. Agree? Or other thoughts?

I think I'm going to pull the clean the idle seats and replace the gaskets (not sure I'm calling them the right thing).

Just realized I never answered one of your questions.
The jeep has acted like this since I got it. The guy I bought it from bought it the same way. He couldn't remember what he'd tried to do to try fix it.
I think that was easier for him to tell me, but it seemed like too good of a deal to pass up. So I bought it.
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Saturday, July 6th, 2019 AT 10:50 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,572 POSTS
Okay, as it was this way when bought I would go back to the beginning. Bring the engine up to TDC on cylinder one and verify the timing mark lines up, pull the cap and be sure the rotor is on cylinder 1. If you had a bad balancer the timing could be anywhere. Next you should use ported vacuum so the timing tracks the carburetor but not a big deal at the moment. If enriching it stops the stumble then it could be the carburetor needs to be gone through and returned to stock, then tune from there. Or find a replacement off a running engine and use it to test.
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Saturday, July 6th, 2019 AT 3:25 PM
Tiny
JDUB80
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  • 52 POSTS
Hopefully, I can get out and try that later. Raining off and on here now.

I've set to TDC once before, but it couldn't hurt to try again. I usually disconnect and plug the vacuum ports (ported and manifold) to adjust timing. (This is correct, right?)

Any tips for setting TDC? I shined a flash light into the spark plug opening until it was close, then held a screwdriver (lightly gripped) until I felt the piston back off and brought it back until it made contact with the screwdriver again. Planning to take a paint pen to the balancer this time hoping to make it easier to see.

How would I know if my balancer is bad?

Unfortunately, I don't know anyone with a carburetor, much less a 4 barrel, and definitely not a truck avenger, so I don't know that swapping with a "good" one is an option.
If you want it the carburetor part number is: 90470-1. Not sure if it'll help anything, but there it is.
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Saturday, July 6th, 2019 AT 5:02 PM
Tiny
STEVE W.
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That will work to find TDC, the main reason is to see if the marks line up, if the balancer is failing the TDC marks will not line up. How is the carb attached? Factory manifold with an adapter or an actual 4 barrel manifold? If it's using an adapter you could hunt up an older carb from a stock 258 to try. Basically trying to eliminate one variable at a time, especially as you don't know who did what to the engine before you owned it. Is the engine itself stock as far as camshaft and compression? Just had the electronics stripped off?
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Saturday, July 6th, 2019 AT 8:55 PM
Tiny
JDUB80
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As far as I can tell engine internals haven't been tinkered with, but I don't know 100%. The fuel pump has been replaced with an electric (near the tank), exhaust looks new (no cat), and I noticed the return fuel line isn't being used (doesn't look like it'd work either.) Other than the electrical and intake, everything else seems to be stock.

I read a post last night (CJMEDEVAC) and he was stressing the importance of the return fuel line and 3 port filter. Made me start thinking maybe I needed to repair mine. Temperatures under hood stay a consistent 110* plus (depending where I checked)
Maybe slight vapor lock?

Thanks again and hopefully you've got lots of patience! This is starting to look like it'll take a while.
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Sunday, July 7th, 2019 AT 7:17 AM
Tiny
JDUB80
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Okay, apparently if I rotate my phone my unsubmitted responses are erased. Here goes a fourth attempt.

My carburetor is sitting on an Offenhauser 4 barrel intake. (Kinda important since I commented that everything not mentioned appeared to be stock)

Had that in my first response, forgot it in the second and rotated my phone to erase it moments ago.
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Sunday, July 7th, 2019 AT 9:06 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,572 POSTS
The return line and filter are a very good idea as carbs can vapor lock. However I don't think that is your issue as it wouldn't keep running like it is, it would run just fine until it got warm, then it would start to sputter and shut down. Once it cooled off it would start to run good again.
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Sunday, July 7th, 2019 AT 2:02 PM
Tiny
JDUB80
  • MEMBER
  • 52 POSTS
Okay, well I figured it couldn't hurt anything so I ordered them earlier today.
I can eliminate that as the issue now though. Thanks
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Sunday, July 7th, 2019 AT 2:04 PM
Tiny
JDUB80
  • MEMBER
  • 52 POSTS
Double checked tdc set it to 8*btdc. It's a lil too late to take a test drive, but I will when I get home tomorrow evening. (One rear park light out, I'll check that out later.)
If it's still stumbling I'll pull the carburetor and start resetting to factory specs, as soon as I can. Should be able to tomorrow, but I've been ignoring the honey-do list for a while now.
Happy wife. Happy life.
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Sunday, July 7th, 2019 AT 8:23 PM
Tiny
JDUB80
  • MEMBER
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Well, got the jeep up to operating temperature last night and went for a little test drive.
Does great as long as I keep my foot out of it, but as soon I try to give it a lil bit (again more than 1/4ish throttle) it bogs and stumbles.

Looks like I'm rebuilding the carburetor soon.

I'll let you know as soon as I'm able to get that done. Thanks for sticking with me through all of this.
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Tuesday, July 9th, 2019 AT 11:04 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,572 POSTS
Be sure to get some of the household chores done. You don't want the Jeep to be the only thing you can cuddle up to. We will be here when you get to it. I would also see if you can find a Jeep forum where someone is running that carburetor, they might have an idea about what it needs if stock isn't it.
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Tuesday, July 9th, 2019 AT 12:53 PM
Tiny
JDUB80
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Definitely gonna keep moms happy!

Thought about joining another forum, just didn't wanna have too many cooks in the kitchen.

Took care of a couple things last night, may have earned back some play time. Lol
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Tuesday, July 9th, 2019 AT 12:57 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,572 POSTS
No need to join unless you wanted too. You could try googling- Jeep 258 with Holley 470 4 barrel - there are a few interesting posts about that carb on Jeeps.
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Tuesday, July 9th, 2019 AT 5:15 PM

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