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Unread 12-24-2011, 10:52 AM   #1
hp_lovecraft
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1981 CJ5 
 
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Location: Maine
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Missing under load

Having read through many threads, they all point to something ignition related.

Setup: '77 232 with HEI and YF.

I figured it was due for a tuneup anyway, so I replaced the plugs, cap, coil, module, rotor. Did not make any difference. Double checked all the wiring, and wired up a relay for the power. Still no difference.

The engine does have a fair amount of blowby. Been that way for years, and have plans for a rebuild, but never had the time as this is my daily driver.

This must be the reason for the misfires under load? The blowby probably reached the point where compression is failing. I'll due a compression test after xmas. What should I look for?

or could it be carb/fuel pump related? I've had my fair share of intake backfires from lean conditions, but can a faulty Carter YF cause misfires only under load?

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Unread 12-24-2011, 01:18 PM   #2
Area.3.Fiftyone
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I would look at changing the plug wires before doing anything else.
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Unread 12-24-2011, 02:52 PM   #3
JeepHammer
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When you get a 'Lot Of Blowby', does the 'Blowby' look smokey?

Have you done a compression test?

Is it a CONSISTENT misfire, like one cylinder laying down regularly?
Or more like a rolling misfire?

-------------------------------------

Now, the 'Fan Boys' of the HEI are going to blast me AGAIN, But did you TUNE the HEI to your engine or did you just plug it in and run it?

The 'Discount' HEI's from the internet/E-bay have WAY too much advance for most I-6 engines out of the box, and that causes detonation problems.

If your 'Miss' is when you are tooling down the highway at speed, and you go to give the engine a little throttle,
Then the engine 'Stumbles', 'Misfires' or 'Dogs Down',
Then you don't have a misfire problem, you have a detonation problem.
Too much ignition timing at the wrong time, and the distributor needs to be tuned before you hammer the pistons out of the engine.

----------------------------------------

If you have a 'Rolling miss' then it's probably lean fuel mixture.

If you have a consistent miss, then it's probably oil deposits on the back side of the intake valve closing off and restricting the intake flow into one of the cylinders in particular.
The cylinder is running 'Lean' when you need extra fuel for power under load, and you are getting a 'Miss' of sorts.
Older engines have bad valve stem seals, the oil runs down the bad valve guides and collects on the back of the hot intake valve...
There the oil gets cooked into fairly hard carbon, builds up and restricts intake flow after enough gets built up...

Low compression on a worn engine doesn't help that situation either....

If you used the 'Super Duper' plug wires that came with the 'Internet' distributors, then those can be an issues also.
I've rarely seen those wires hold up for more than about a year, and I've seen them fail testing right out of the box, give up in a month or two, ect.

If you cap, rotor, plug wires and plugs are all new, and you are STILL getting a miss, let me know EXACTLY under what circumstances you are getting that miss,
Throttle position, load, what you were doing when it starts to miss, ect. and I'll try to help.
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Unread 12-24-2011, 03:30 PM   #4
hp_lovecraft
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Quote:
If you cap, rotor, plug wires and plugs are all new, and you are STILL getting a miss, let me know EXACTLY under what circumstances you are getting that miss, Throttle position, load, what you were doing when it starts to miss, ect. and I'll try to help.
In my case, if I start, and drive with fairly little throttle, I do not notice any misses at all. But if I open the throttle up beyond say 1/2 full, it starts to miss in 3rd or 4th gear. This usually happens if going up a hill, or trying to accellerate.. The misses seem inconsistent, unless I'm at WOT then it seems worse like a cylinder is consistently missing. (like when a modern car hits the rev limiter)

It also misses if I lug the engine down to 500rpm or so.

I'm not a HEI fanboy. I bought one simply because my prestolite was failing (and was a mess to begin with thanks to the PO). Its worked for the last couple of years. I have not replaced the wires because they seem fine. Bought them at advance or autozone.

I'd be happy to replace them, but any way to test them? I guess my next step is a proper compression test. I still wonder if the problem is in the ignition. The wear on the contacts looks very odd. There is burn marks right on the very edge of the contact, but not in the middle like it usually is. I'll get a picture tomorrow.

I don't think its running lean. I've had lean problems before (bad vac leak, and bad timing). Both times the engine ran wicked hot. Engine is running cool now.
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Unread 12-24-2011, 04:07 PM   #5
JeepHammer
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Quote:
Originally Posted by hp_lovecraft View Post
In my case, if I start, and drive with fairly little throttle, I do not notice any misses at all. But if I open the throttle up beyond say 1/2 full, it starts to miss in 3rd or 4th gear. This usually happens if going up a hill, or trying to accellerate.. The misses seem inconsistent, unless I'm at WOT then it seems worse like a cylinder is consistently missing. (like when a modern car hits the rev limiter)

It also misses if I lug the engine down to 500rpm or so.

I'm not a HEI fanboy. I bought one simply because my prestolite was failing (and was a mess to begin with thanks to the PO). Its worked for the last couple of years. I have not replaced the wires because they seem fine. Bought them at advance or autozone.

I'd be happy to replace them, but any way to test them? I guess my next step is a proper compression test. I still wonder if the problem is in the ignition. The wear on the contacts looks very odd. There is burn marks right on the very edge of the contact, but not in the middle like it usually is. I'll get a picture tomorrow.

I don't think its running lean. I've had lean problems before (bad vac leak, and bad timing). Both times the engine ran wicked hot. Engine is running cool now.
Sounds like you have two issues,
One is rotor phasing,
The rotor is on one side of the plug terminals when the ignition fires, then the spark has to 'Jump' to the edge of the terminal it's already passed, or is coming up on....

You can EASILY fire the wrong terminal when this happens.

Also sounds like you might have an aluminum terminal cap if the firing pattern is that obvious.
Aluminum burns up into aluminum oxide when high voltage hits it, and becomes NON-conductive for high voltage transfer.

There is a reason I preach BRASS TERMINALS, they don't burn up like that and become non conductive.

Rotor phasing problems happen when the rotor is out of 'Sync' with the plug terminal when the ignition fires...
As vacuum advance comes in, the rotor gets farther and farther away from the proper plug terminal, and that drives the firing voltages WAY UP, unnaturally high.

When it has to jump a big gap, all the energy the coil is producing has to go to voltage to ionize that huge gap, and there isn't anything left over for Amperage (Heat) or Duration (The time the spark lingers in the gap).

Rotor phasing usually has two common issues when it's way off,
One is the vacuum advance is hooked to manifold vacuum, yanking it around to full advance virtually all the time,
And the second is the distributor gear is usually on the wrong side of the shaft.

These distributor gears have 13 teeth, and normally the 'Discount' gear places don't pay much attention to where the gear needs to be to keep the rotor aligned...
(Most of the 'HEI Clone' makes have never heard of rotor phasing, so they have no clue... They are just assembling 'China' parts and not paying attention to the results).

When you go up a hill, or try to accelerate slightly in traffic,
Your fuel mixture is at it's LEANEST,
If you have large tires, your engine has slowed down much more than it was intended to at this particular MPH, so you are having to 'LUG' the engine more...

Add in a bunch of timing, you get a 'Bog' or 'Hesitation' people often call a 'Miss'...
It's simply too much load, too lean of fuel mixture, and too much ignition timing...

If you wan to prove it to yourself, take the vacuum line off the distributor and plug it,
Then go lug the crap out of the engine someplace to see if the 'Problem' happens.

IF your issue goes away, then you found out there is too much ignition timing staying in the curve WAY TOO LONG...

If you tune the distributor to DROP timing when you crack the throttle blades going up a hill, or trying to keep up with traffic in high gear,
Then you won't have those issues....

Most people jack in WAY too much 'INITIAL' advace, 6 degrees is plenty for a good idle,
And then they hook these things up to manifold vacuum, which usually pins the vacuum advance in FULL all the time.

The vacuum advance is supposed to 'FLOAT' with load.
When load increases, vacuum drops, and the vacuum advance is supposed to drop some timing.

If it doesn't, you get a 'Bog' or 'Miss', when it's mild,
And when that detonation gets really bad, you get 'Spark Knock'...
The engine won't live long if you get 'Spark Knock', but even the 'Bog' or 'Miss' will damage the engine over time...

Do you have any idea what vacuum signal you are pulling at PTC (Part Throttle Cruise) before you tip into the throttle a little?

What does the vacuum drop to when the 'Miss' is most prevalent?

-------------------

Once you know where the vacuum threshold is at PTC,
Then you can use a hand pump, vacuum gauge & timing light to set up your vacuum advance to DROP some timing in that range, and your 'Miss' will reduce or probably go away...

----------------------------

As for plug wires, GOOD ONES will come with a spec sheet that tells you how many 'Ohms' per foot of wire.
(Ohm, a measure of electrical resistance)

That's a start.

Personally, I slap a good set of wires on to start with instead of the 'Replacement' wires made in China you get from the parts stores.
Even if they cost you $100, that's $10 a years for the next 10 years you don't have to worry about them,
While the 'Discount Sore' wires will fail anywhere from out of the box to about a year... and they run around $50 anyway, so they aren't cheap...

I test wires under load, but not everyone has a distributor machine...
I can usually load the 'Replacement' wires enough to make them fail completely, or catch them on fire!
You can't do that with an MSD set of 'Cut To Fit' wires with good wire, good terminal and good boots...
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Unread 12-25-2011, 10:26 AM   #6
hp_lovecraft
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Quote:
If you wan to prove it to yourself, take the vacuum line off the distributor and plug it,
Then go lug the crap out of the engine someplace to see if the 'Problem' happens.

IF your issue goes away, then you found out there is too much ignition timing staying in the curve WAY TOO LONG...
I unhooked the vac advance to see what the difference was. It still misses under load, but now it has no power. I was unable to get it above 25mph. However, I believe I'm using ported not manifold vac. Hard to tell on the YF, but its the "distributor port" on the carb.
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Unread 01-01-2012, 12:55 PM   #7
hp_lovecraft
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Just to follow up- I got new plug wires, and rebuilt the carb. Neither made any difference (but both were due regardless).

Here is a picture of the odd pattern on the distributor cap terminals:



You can see very slight contact marks on the very edge of the terminals. This is different. The marks used to be spread across. This might imply the distro is out-of-phase? or, it might just mean I don't push the jeep under load, so the timing never has to advance itself.

I did notice a slight amount of play on the rotor. No more then 5-degrees. However that wouldnt effect phase since it still only going to fire when the relector is in the right position?

For a test, I could dig out my old prestolite and hook it back up just to see if the problem goes away. That would isolate the ignition system as the problem. Otherwise something else would be causing the lean misfire under load?
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Unread 01-01-2012, 02:31 PM   #8
JeepHammer
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Only firing the extreme edge of the terminal like that means you rotor isn't pointing in the correct position, and as the vacuum advance come in, the rotor is getting further away from the terminal,
That's a rotor phasing issue, which you CAN NOT fix on an HEI other than to turn the distributor gear round 180 degrees and see if some of it clears up.

Testing with a different ignition helps, and also suspect the module or trigger for failures in the HEI,
And switching distributors will find those issues pretty quick...

Do you still have the 'Used' or a Junk HEI cap laying around?
If you can use that old cap, cut a hole in next to the #1 terminal, so you can see the rotor with a timing light, you can check to see if rotor phasing is the issue, or one of the issues, or not...
This gives you VISUAL conformation of rotor phasing...
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Unread 01-01-2012, 03:30 PM   #9
hp_lovecraft
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Quote:
Originally Posted by JeepHammer View Post
Only firing the extreme edge of the terminal like that means you rotor isn't pointing in the correct position.
But what would cause that? It did originally work for a couple years. Cheap/worn reluctor? There doesnt seem to be enough parts in an HEI for something to fail without being obvious.

This might explain why it could not get above 20mph when I disconnected the port-vac. It was too far out of phase that even the mech/vac advance could not put it in phase?

Quote:
And switching distributors will find those issues pretty quick..
Unless those other distributor is a Prestolite that the PO had "rigged" up to work. Its a mess of wires!

Bottom line is that if the phase definetly looks like the problem, and its not fixable because critical parts are worn/broken, I'll probobly just get a new distro. Its getting into the time a year when I'd rather not be doing work on the jeep!
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Unread 01-01-2012, 06:13 PM   #10
RenoF250
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I think JH has you on the right track but just in case, what do you mean by miss? Does it chuff at all? I am thinking you may have a bad intake valve and when you get on it it is mildly backfiring into the manifold. Is that possible?

Also, the rotor will have play, that is the mechanical advance.
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Unread 01-02-2012, 02:23 PM   #11
hp_lovecraft
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Its not backfiring up the intake. I've had those problems before (bad vac leak, and loose distributor). It feels simply like some of the cylinders stop firing when its under load at higher throttle. Typical ignition-related problems.

The phase definetly seems to be an isuse. Here is a pic of the old cap that I've used for a couple years:


You can see the wear on the terminals is more uniform. Could be unrelated, but replacing the HEI would be easier and cheaper then pulling the head for valve work!
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Unread 01-08-2012, 01:30 PM   #12
hp_lovecraft
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Just an update- I went out a got a nice vacuum gauge. I figured they would show clear signs of a burned out or sticking valve.
Hooked it up, started the engine, and got a PERFECTLY STILL reading of 20". What the heck? That was fast idle. When it warmed up, it dropped to 18" but bounced a tiny bit from 17-18". Thats expected as I have some blowby.

So I thought about Jeephammers PHASE Test above. I have a spare set of HEI internals.
Then I had a different idea. Why not take the rotor, and BEND the contact over about 1/2 centimeter. So I took my mini-torch, melted the end of the rotor, grabbed the contact and moved it. The plastic is all melty, and cracked. But it should be able to tell me if I had a "phase" problem.

So I dropped it into the HEI.... and the MISSING UNDER LOAD problem completely disappeared. I tried opening the throttle in high load/low load/different gears/wot/up hills/, etc. No misses at all.

What does this mean? Well, I assume it means the distributor itself is bad? It used to work just fine, so I don't understand why the phase could be so far off now? It doesnt seem like something that can change in an HEI. Unless the problem is actually DWELL from the module? I know bad modules can produce incorrect dwell giving the same symtoms. Though the first thing I did was swap out the module with the same result.
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