CJ5 4.2 Timing/Backfiring problem - JeepForum.com
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post #1 of 15 Old 08-05-2020, 08:22 PM Thread Starter
JeepinGeo
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CJ5 4.2 Timing/Backfiring problem

Hey all! I am new to these forums, so I will try my best to explain my troubles. For starters, I have a 1977 CJ5 4.2L with a Ford T18, Dana 30 up front, Dana 20 Tcase, and AMC 20 rear. 3.54 gears in both (4 wheel drive works). It's got a 3" body lift, no suspension lift, and rolls on 35" swampers. I've only owned the Jeep for 2 years now as it's my first vehicle. It was abused when I got it but I am slowly making repairs. As far as I know the engine has a mild cam, headers, HEI dizzy and a Weber 38 carb. This is very difficult to explain but here lies my trouble:

I start it up and it sounds like it has a misfire, at least from cylinders 1, 2 or 6.(dual exhaust, 1, 2, & 6 are on driver's side & it sounds horrible). Replaced spark plugs and still runs like crap (NGK V-power). At running temp & warmed up, under any load the engine bogs down and acts like it wants to die; begins backfiring. I get to a stop sign or stop light once it's warmed up and the engine drops to a real low idle before it stalls out if I don't give any immediate gas. My dad & I have replaced the cap and rotor, spark plug wires, and spark plugs. After a few weeks we checked the plugs again, all of which looked brand new still. We set timing to 8 degrees BTDC and adjusted mixture screws/idle screws to run its best at 650-700 RPM. For a month or two (since I've been driving) it ran great. All hoses and lines appear to be connected properly. No exhaust/intake/vaccum leaks from what I can tell. At all times, however, the timing is advanced. The arm under/next to the distributor vaccum pot seems to always be advanced, even at idle - Don't know if that is correct or not? The dizzy vaccum advance is ported off the carb, not manifold. While setting the timing, it always seems to fluctuate just a little bit between 7 and 9 but we've just gone with it. Tomorrow I will be doing a compression test and possibly a leak down test just to get a good idea of the engine's condition. Will also try to check manifold pressure too. Since day one I've run Rotella 15W-40 in it with Lucas stabilizer. Seems to love it with a constant 30-50 psi oil pressure. Head gasket still seems to be intact too; no coolant in oil/overheating, etc. No idea how many miles are on the engine though - speedometer was missing when bought it, and the jeep originally came with the 304, not 258.

What it comes down to:
Spark plugs - checked and good
Plug wires - checked and good
Cap & rotor - checked and good
Carb hoses - checked and good
PCV lines - checked and good
Fuel lines/filter - checked and good

Potential culprits:
1. Timing chain potentially stretched and screwing with timing?
2. Distributor base unit/vaccum pot bad?
3. Ignition coil bad?
4. Potentially my fuel pressure going into the Weber 38? It's direct from the mechanical pump straight to the carb - probably 7 to 8 psi. I don't have a pressure regulator hooked up currently.
5. Sticking valves?

Really can't think of anything else that would be causing this off the top of my head. I may have confused everybody, but basically I have had this Jeep since October 2018. I didn't get my license to drive until September 2019. Had a few incidents with my axles (bad axle bearings caused my axle/wheel to actually spin out and roll down the road - story for another time) and a lose tcase yoke nut that backed off from not being tight, causing the yoke to slip off and my driveshaft to go flying which grenaded my rear axle (eventually rebuilt). With all that said, I only had a good 2 months of nice, comfy driving until this new issue appeared. Just not sure what's going on. I've read forum after forum and am still racking my brain. This seems like a normal problem but everything on this engine feels jacked to me. I feel bad for making ya'll read all this, but maybe ya'll can help me gain some insight into what's really going on. Any advice would be greatly appreciated! I respond quick too. Thanks.
- Geo

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post #2 of 15 Old 08-05-2020, 09:06 PM
Timaaaay83
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Wish I were you. Late teens doing a 40+ year jeep. Unfortunately in my mid 60's doing a frame up. No advice from me but you've come to the right place. Members have helped me through a lot of troubles, even from 10 year old posts. Good luck and keep us updated.
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post #3 of 15 Old 08-06-2020, 06:26 AM
Matt1981CJ7
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First off, 35" tires and 3.54 gears is a terrible combination. 31-32" tires would put a lot more pep in your CJ, and would be easier on your engine and driveline. Or, if you're enamored with the big tires, you should re-gear to 4.56.

I've never seen a 4.2L I6 engine with dual exhaust. Could you post a few pics of that setup?

Anyway, let's start with the backfiring. Is it a loud explosion like a gun shot, or is more of a popping sound? Is it in the exhaust, or thru the intake?

Matt


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post #4 of 15 Old 08-06-2020, 08:03 AM
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My 4th CJ-5 (1979) had a 2 piece header with dual exhaust.

If your engine is original? your VIN might tell it's story.

1977 had a 232 and a 258 straight 6

Both had 1 barrel carbs and 1 barrel intakes

Their cam differed from the later 2 barrel 258

Tuning specs differ for all

Did someone just slap a 2 barrel intake onto whatever was there?

Are the mix screws are both the same # of turns out?

---JEEPFELLER
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post #5 of 15 Old 08-06-2020, 12:27 PM Thread Starter
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The backfiring is definitely through the exhaust and not back through intake/carb. It's more of a popping than a gunshot, but it's constant once warmed up and/or under load. I'm assuming that would indicate a vaccum leak rather than a sticking exhaust valve...? Also I am well aware of my tire size/gearing issue; just don't have enough money right now to throw different tires on it or swap gears as I'm still in school. However I don't dog the jeep and it has not seen much offroad, although I wish. As far as the intake goes, it's an aftermarket aluminum 2 barrel manifold. Once again, I did not install it - the PO did. Headers are in fact two piece (1,2,6 & 3,4,5) and split off to each side at firewall. Not sure what it's from or what year. Idk if I mentioned it either but the Jeep originally came with the 304...PO swapped it out and installed this 258...no idea why. I know it's difficult to diagnose without hearing so I understand the different questions ya'll have - don't be afraid to keep asking. Also don't think I can upload pictures on this forum account. Thanks.
- Geo
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post #6 of 15 Old 08-06-2020, 12:29 PM Thread Starter
JeepinGeo
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Mixture screws are both turned out equally BUT I did just notice that the whole engine is kind of angled more one way than the other in the engine bay...must mean the mounts are off down below, which I have never checked until now. Could that be causing one bowl to fill more than the other? Thanks.
- Geo
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post #7 of 15 Old 08-07-2020, 06:29 AM
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My guess is you have a significant manifold leak. The manifold bolts on a 258 are renowned for loosening up, especially the rear hard to get to bolts. Since you have a atypical aftermarket manifold setup, that's where I'd start looking.


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post #8 of 15 Old 08-07-2020, 09:24 PM
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Quote:
Originally Posted by Matt1981CJ7 View Post
My guess is you have a significant manifold leak. The manifold bolts on a 258 are renowned for loosening up, especially the rear hard to get to bolts. Since you have a atypical aftermarket manifold setup, that's where I'd start looking.





Matt
if this is the case or leak, loose manifold bolts, is it a matter of just tightening them up or does the manifold have to be removed, new gasket, tightened back up, or ... ?

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post #9 of 15 Old 08-09-2020, 09:58 AM
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I would do the new gasket thing,

Finger tighten, then maybe 10 ft.lbs. (in the sequence of the manual),

Then finish off with 25 ft.lbs. (again, in the sequence of the manual) (don't just crank 'em down! The manual tells you "which to start with" and which bolt is "Next"--they are numbered).

A few years back, a bright, young, college feller got wind that I might have a 2 barrel intake. After talking with him a little while when he came for the part, we figured out that he would break this one too without a little coaching.(Starting on one end and working your way to the other will break one every time!)

We gathered up some tools and the intake and met him at his (parents) house. Then we made him install it. I even posted a copy of the sequence right in font of him.

Pretty much that little bit of Jeep schooling got him investigating and doing all of his mechanicing right. It's been over a year ago that he installed a crate 258 in his Jeep, all by himself!
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post #10 of 15 Old 08-09-2020, 10:14 AM
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I agree that it could very well be a manifold air leak. I ran a set of headers on my 258 several years ago and the header/manifold bolts were always loosening up causing air leaks. The air leaks would affect idle and at times cause backfiring...popping through the exhaust. The headers I used were Advance Adapters and it had a thick common flange with intake ports cut in it. The intake manifold bolted over the header flange and the installation used thick gaskets between the flange and head and intake and flange. So two thick gaskets that would compress over time resulting in loose manifold bolts and air leaks. Eventually got rid of the headers and put the exhaust manifold back on. No more intake leak problems.
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post #11 of 15 Old 08-10-2020, 09:10 AM
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Quote:
Originally Posted by JEEPFELLER View Post
I would do the new gasket thing,



Finger tighten, then maybe 10 ft.lbs. (in the sequence of the manual),



Then finish off with 25 ft.lbs. (again, in the sequence of the bolts .....
so, when you say "I would do the new gasket thing" you're saying basically strip down the right side, remove the manifold, replace the gasket and tighten by sequence ... NOT simply tighten the existing gasket, tightening bolts in sequence?

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post #12 of 15 Old 08-10-2020, 10:59 AM
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Correct!

Maybe "my pal" Tucker put it (yours) on to begin with (feller discussed up above) before he sought out help at my house.

Not only might the gasket be unevenly squished or tore, re-tightening what you have might be "reckless" maybe sorta just like squirting silicone around everything.

My boy Tucker took it one step further, he busted the intake (a nice chunk out of it) with his one end to the other "get 'er done" sequence,

Taking it all down is going to let you thoroughly inspect stuff (even for a small crack-unseen from the topside) Clean your surfaces well (as you have no clue what "Tucker" did when he stuck it together).

The only difference here is the cost of a gasket or two, more time, pride in your work.

When you are done, you can say this was done correctly, not just a patch job, I have eliminated a possible vacuum leak at the manifold.

Just as a nice touch (I'm not pushing it on you!) with this one and previous CJ-5s that I had, I installed studs in the bolt holes, I'm pretty sure that 2 in the center and low posed an accessibility issue so they remain "Bolts". The studs kinda hold the gasket and manifolds (or header) in place while you work. To me, it's EZer to put a nut/ washer/ lock washer on where you can see it, than to fish a bolt around in a blind spot while juggling the manifold around. If you ever attempt this, use studs that have the same coarse threads on either end. Fine thread actually "push" harder (due to mechanical advantage) You will over-do your correct torque specs. If you are some sort of engineer you might figger out how much to back out of the normal torque. Just seems EZer to use the fully coarse studs! Sometimes you have to think this stuff out and you never want to bind anything! My 258 has a 4 barrel intake and headers.

-----JEEPFELLER
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post #13 of 15 Old 08-10-2020, 09:48 PM
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I skimmed back thru the thread.

Are you timing with a timing light at the correct RPM or are you kinda doing it by ear?

You say the engine is angled----front sloping towards rear? Carb sits on the manifold with both mix screws towards the radiator?

I can follow your concern.

With my Holley 390 on my 258, my tangible symptoms were mostly a horrible idle, I knew the float/fuel level could only be adjusted for one or the other venturi (in the middle would not be correct), along with that lower performance than it should have, just that "something ain't right" feel about it.

BUT it Never backfired at all!

The unit sits sideways on the 4 barrel manifold (one tank on the drivers side fender/ the other tank closest to the engine).

My engine is high on the front and low on the back. My venturies would also be high and low. I tried compensating with a one or two size jet difference on either side along with trying to compensate with the mix.

I was not satisfied.

I felt something correct need to happen, not attempting to band-aid the situation

It was not long before I had a machinist take a one inch aftermarket spacer along with very accurately measuring the slope. For $45 he got my slope correct and left what he could of the spacer. (one of my stupid thoughts was to grind it with my grinder and save $45!)(Yeah right!)

My 1st pic is the carb on a aluminum spacer in the beginning (this spacer was sorta hollow and unfit to machine). Pretty much you are unable to see the slope of the manifold in this front view.

I just ran out there with my phone camera, my canoe kinda limits how high I can raise the hood and its dark out there (I got a little scared!)

So this pic shows the black plastic 1" spacer, below you can see the engine/ manifold slope-age and above it you can see how level my carb now sits. It works so good now!

-----JEEPFELLER
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post #14 of 15 Old 08-11-2020, 09:16 AM
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hello

when you do the manifold gaskets. i use the felpro one just because it also comes with the oring and gasket for the heater under the intake manifold. i have seen them leak because they are so old and forgotten. also after i install manifolds and new gaskets i run it for about a 100 miles then i go back and do a re-torque on the manifolds in sequence. make sure that you use the cone shaped washers on all the bolts and studs. the smaller ones on the center of the exhaust manifold. when i bought this cj7 i have now the PO didnt have one on the back stud. it was leaking when i bought it. the nut was barely holding any of the manifold. the gaskets are a paper and will compress from heating and cooling cycles. i get a few years out it. most of the time for me its about 3 years and have to re-gasket the weber adapter. last time it was whistling dixie when i let off the gas.

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post #15 of 15 Old 08-11-2020, 09:32 AM
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oldschool, thx for that headsup re: the gasket.

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