Help, I have lost 1 cylinder

[yuza]

I am running a 4.0 HO swapped engine with intake and carb from the original 258
Today (and occasionally in the past weeks) it seems to miss 1 and sometimes 2 cylinders
I replaced spark plugs 2 weeks ago (but they are full black already...I need to tune the carb)
I checked all plug cables and they seems ok (PO repaced them) but I have a doubt about the distributor: PO didn’t know engine was replaced and he did install a new distributor for a 258. Should I install a distributor for a 4.0?
It seems to be somehow not matching properly

Any idea of what the cause of misfiring could be? Or what part shall I replace first?
Many thanks

[Cecil14]

If compression is good in each cylinder, the distributor is about the only thing left in the mix to be causing problems. If you are having problems consistently with one cylinder, it would have to be the leads on the cap for that cylinder.

Couple things I would check first:

• Compression: Make sure you don't actually have a dead cylinder. A leak down test would also be a good idea.
• Distributor cap and rotor: get a good brand cap and rotor with brass contacts.
• Wires: might be worth changing them out altogether, but that's pretty pricey for an unknown. I would try swapping cables on the suspect cylinder with a known good one before buying new.
• Distributor itself: make sure it's not loose, and that there is no play in the shaft. A worn out distributor could present with your symptoms. Set it to TDC and adjust timing from there so you know it is correct.
• Plugs: fouled plugs are definitely not helping you. I would run the least expensive plugs you can until you get things tuned up correctly. No point fouling out expensive plugs trying to diagnose a problem.

You definitely need to get the carb tuned in better before going much further. You should be able to get it a lot closer than it is even with it running a bit rough. What carb are you running?

To answer your other question, any 258 distributor should fit the 4.0L. The block dimensions are very similar. I ran an HEI distributor on my first carbed 4.0L for quite a while with no problems.

[yuza]

Thanks mate. Carb is a carter BBD
The problem is intermittent. Was running good for months, then one day early (and humid) morning was barely able to idle and was undriveable. Replaced plugs and it was ok
One week later it happened again until engine warmed up, then run ok. Last weekend was running smooth, cold and warm, today again same problem all the time no matter engine temp.
I have no tool to check compression but I would exclude it
I already have a carb rebuild kit from Walker and my intention is to have it rebuilt by a pro, since I am a decent motorcycle mechanic but not familiar with complicated 2 barrel carbs
Also no spare leads available and not easy to get replacements quick, I live in Italy and spares shops here do not stock these parts for old jeeps of course
I will follow your recommendations as much as possible anyway, starting by understanding which cylinder/plug is faulty and eventually try to switch the leads.
I can buy online cap, leads etc from ebay but need to wait few days for delivery

[Cecil14]

Pull your cap/rotor and post a pic of the underside. Make sure that the contacts are nice and shiny.

It's possible you have a weak coil as well, though it doesn't sound a lot like it to me.

For the carb...good luck. The BBD is not a great carb. Not extremely complicated, just not great. Reliability is not something they are known for. A motorcraft 2100 or 2150 would be a MUCH better choice, but I imagine hard to come by over there.

Let us know what you find.

[tgreese]

If the problem seems connected to humidity, I would first suspect the ignition wires. Owners usually let the wires go for years, and even though they look good, they wear out. Note that the engine temp coming up can dry out the ignition parts, so the problem could be more about moisture than the temperature. Have you ever replaced the wires or the rest of the ignition parts like cap and rotor? This is a Duraspark ignition, and condition of the connectors is critical. See this thread - viewtopic.php?f=10&t=17734

[yuza]

I didn’t replace anything in the ignition system since I own it, last april
It started having misfires and poor idling occasionally since the weather got colder and more humid. Today wasn’t humid at all btw but overnight for sure it was
Leads and cap seem to have been replaced recently and the PO also told me he did but not sure how long ago
Coming back to testing presence of spark, and ideally identify what cylinder has none, any better method than touching the engine with the plug?

[Cecil14]

You can use a timing light, if you have one. Clip the lead to any of the wires and see if it triggers the light. That won't be a perfect tell, of course, as it doesn't take nearly as much power to trigger the light as it does to fire a spark plug. I would also compare the plugs, does one look considerably different than the rest?

[fulsizjeep]

It wouldn't hurt to look for vacuum leaks while you're at it. The 258 had a whole bunch of vacuum hoses by 79. Those BBD carbs are crap in my book. The 81 J10 258/BBD I had was nothing but trouble with vacuum leaks and a crap box carb when it was only 4 years old. Not to mention the PO had removed some of the emissions. I eventually stripped the rest of the emissions and replaced the BBD with a Holley 350 2bbl for a much more pleasant driving experience.

[yuza]

No timing light available unfortunately
And also in my case the PO removed many of the vacuum hoses, but the one from carb to the distributor for the timing advance control is there

[yuza]

Another question: i am running an electric fuel pump that is rather noisy, particularly after having run out of fuel a couple of times, and I am thinking of replacing it. Any recommendation on brand/model? Do you know the specs I should be looking for?

[yuza]

Doesn’t look worn or broken to me

[yuza]

Tested all spark plugs with the old school method (by touching engine block) and I have sparks on each cylinder. Some brighter than others but no faulty plugs apparently
But it stills runs bad, and all plugs are blackened
Ordered a set of leads meanwhile

[letank]

cap and rotor looks good enough, make sure that the choke is properly adjusted, you can test the goodness of the spark plug wires with an voltmeter, there is a specs for the resistance of the HT wires, usually no more than 9K ohms

these instructions could be of some use

https://www.4wdmechanix.com/Rebuilding- ... uretor?r=1

EDIT: download the service manual from Tom's site, year 79 or 82 has very good instructions for this carb

http://oljeep.com/edge_parts_man.html

[4bz]

If there is any scale on the terminals inside the distributor cap, scrape it off with a sharp knife. Aluminum terminals will build up scale in humid weather, and the scale is not a good electrical conductor. I cannot tell from the photo of the distributor cap if you have this issue or not. A heavy scale buildup will generally come off in flakes that you can easily see, and your engine will run much better afterward. Distributor caps with brass or copper terminals do not have the oxidation problem that creates the scale.

[GrandW]

Sounds like timing since you say your getting a spark to all plugs, all plugs are fouled with gas (so it's getting fuel), and it 'kind of runs' which indicates air. If you can pressure test it as someone mentioned, that'd be great, but if it's got all 3 keys and just not working together as 1, high chance it's timing is off. The timing light would be great for this.

[yuza]

Thanks all
It is now in the hands of a (hopefully) good mechanic, carburator expert it seems...he is currently rebuilding the carbs of a range rover 3.5 mark1
I guess he will do all the proper testing
And new leads and fuel filters are on the way
Can the engine lose the right timing for any reason? It was running fine just 2 weeks ago, despite surely overfueling

[yuza]

Would it make any sense, in case I have to replace the distributor, to go HEI?
I have read quite a lot on Hei conversion but it was mostly for the 360 with points, I have not understood if it is beneficial and how much on a 258 with electronic ignition

[tgreese]

Would it make any sense, in case I have to replace the distributor, to go HEI?
I have read quite a lot on Hei conversion but it was mostly for the 360 with points, I have not understood if it is beneficial and how much on a 258 with electronic ignition

I would not. It's a sideways move, not an improvement IMO. You have a Ford Duraspark distributor, and it's basically an "HEI"-type distributor made by Ford instead of GM. You can use the GM module with the Ford distributor and vice versa - electrically the same.

The Jeeps with Duraspark can be upgraded to use Ford parts - a big cap/rotor, HEI-style wires and a hot coil, which then includes the "high energy" part of HEI that the Jeep ignition does not come with. Search for "TFI upgrade." Jeeps come with a conventional cap and coil, for some reason - Jeep's cheapness or Ford's reluctance to sell their best system? Don't know, but you can add parts to make Ford's premium system. Even with the present style of cap and wires, I would get a "premium" cap with brass terminals rather than aluminum.

There have been a lot of issues with cheap replacement HEI distributors damaging their engines. For a six, you can use a Chevy 250 HEI distributor with an AMC V8 distributor gear, but I think that's not done much today. Most would just go Duraspark with Ford parts or buy an aftermarket HEI distributor for the AMCs.

If you want to upgrade your ignition generally, I would suggest the TFI parts mentioned above and replace the Duraspark module with an MSD capacitor discharge module. You can trigger the MSD module from this distributor trivially, and it will give you a much hotter and longer spark, superior to the HEI or hot-rodded Duraspark.

Why do you think you might need to replace the distributor? You could "lose" timing by having the timing chain jump a tooth, usually called "jumping time." This would be a lot worse than the engine missing occasionally. Otherwise the timing will be stable - this is one of the merits of electronic ignition. You could have a problem with the electronic function of the ignition that could lead to poor performance, and this is covered in the TSM.

[yuza]

Thanks, very clear now!
Sorry for asking so many silly questions but I am very new to jeep world and I am the kind of person who can’t sleep if anything in any of my vehicles is less than perfect!
It is a costly attitude....lol
Anyway, few chances of finding ford parts over here to do the conversion, so as you suggest I might just go for a brass contact distributor cap and rotor as a starting point eventually

[TheBeefChief79]

A friend had a similar issue and even though everything seemed to be good when you turned off all of the lights and had it dark you could see a plug towards the back that was creating an arc off the engine block when it was running.