Yes it's winter but I'm not pining for summer.
I have a miss that happens when Jessie warms up but not when cold.

86 Lucas V12.
Starts up from cold beautifully. Immidiately.
Idles at 950-1000 cold. Warms up quickly.
As it wars idle slowly drops to 750-800.
Perfect steady idle cold.
As it warms it starts to miss on occasion.
Once warm it continues to miss periodically.
The misses are soft poom sounds like the mixture is firing somewhere outside the exhaust valve. Not loud bangs just these pooms.
VrrrrrrrrrrrrrrrrrPoomVrrrrrrPoomVrrrrrrPoomPoomVr rrrrrrrrrrrrrrrPoomVrrrrrrrrrrrrrrrPoomVrrrrrrrrrr rrr

New primary coil
new leads
new rotor
new cap
newish plugs
Pulled them, all dry and tan colored. Gapped to .25
New fuel pump
35psi at idle
No A bank regulator
New CTS
New ATS
TPS at .35V.

 

My thoughts before lunch is served.

Do you have the over-run valves in the front of the inlet manifolds??, if so, remove and blank plate them. Many strange mis-fires have been linked to them.

The air pump is also a culprtit, so also, remove it if allowed, we are.

Inlet manifold nuts have loosened off a tad. Run a 13mm socket over all 24, and ensure thay are snugged up. NOT stupid tight, just common sense tight.

Lower injector seals ae leaking vac. That is the square section small ones at the tip.

 

i knew there was more pertinent info I was leaving out.

Overun valves deleted
air pump deleted. Inlets tapped and plugged.
New injector seals and bushings.
Injectors all firing nicely, no drips.
Original injector harness in good shape. Boots unbroken on connectors. wires flex without any cracking. Two crispy areas under ( deleted ) A/C compressor.

Now those intake bolts... I haven't checked them. I'll find a torque spec and do them up.

I have a new set of plugs on hand. Is there any chance that this could be caused by a couple of plugs with their electrodes shrouding the spark? I recall a verse in the book abhout aligning them so they face the swirl.

 

OK.

The plugs are my next on the list.

I run NGK BPR6EF in all the HE engines. I used to run BP6EF, NO resistor, but NGK stopped making them about 12 months ago, so the "R" is all we can get.

I know the book/s all call for a flat nose (non projected) spark plug, (BR6 ???)but that shrouds the spark from the bit what needs to go BANG, and since I have been using the "P" spec plugs those annoying misfires are no longer.

Also, the idle "pot" trimming of the ECU may need a click or 2 to get that spot on.

 

Sounds like a failing ignition module to me. Use only the Gm/Delco module, # D1906. All the aftermarket ones I tried are inferior to the GM original.

The V12 pushes the module right to it's limits, so what might work "good enough" in a V8 has it's shortcomings exposed in the V12.

 

Might be worth putting the TPS to 0.32, which is spot on the spec too. Also checking and cleaning the coil low tension connectors, tightening up the throttle body to manifold bolts, checking TB and butterfly for goo buildup, butterfly gap settings, and then checking all the many vac tubes and other tubes, and blanking gromets that go to the manifold tappings for tiny leaks and perished rubber. It will easily be caused by something like that. Including cleaning and checking the crankcase ventilation gizmo, and its manifold connections starting behind the B bank airbox.
Greg
Greg

 

I've got three here. Original, Echlin TP45, and a Durmast DR100. All equivalent according to the following cross reference, but I'll look for a D1906. I haven't been able to notice any difference in the three I have.

http://www.wgcarbs.com/forum/index.php?topic=943.0;wap2

AC DELCO -------------------- DM 1906 or D1906
WELLS ------------------------DR 100
ACCEL -----------------------ACC 35361/ACC 35367
PERTRONIX -------------------D2000
CAR QUEST -------------------21040
NAPA ECHLIN -----------------ECH TP45
NIEHOFF ----------------------DR 400
BORG WARNER ----------------Select CBE4/CBE22
GM----------------------------10482820
Mallory-------------------------607
Jegs---------------------------555-40600
Moroso-------------------------97857

 

I can reset the TPS. Maybe it will stick, it can be .1 different if I hold my mouth different.
I checked the coil connections are good, but I want to relocate it and I'll replace the connections when I do.
TB bolts, will check.
No goo on my TBs or butterflies and they are gapped correctly.
Throttle linkage is slick, smooth, and properly set.
All vac lines have been replaced in the bay. May have leaks under dash.
ECU vac line is clear and sound.
All vacuum caps are new heavy solid rubber style.
Do need to renew the AAV tube.
PCV valve is loose in its air cleaner grommet. Not a vac leak, but needs replacing. How do I test this piece? Just swap?

So, as soon as the festivities here are over I'll...
Tighten up the entire intake track connections and check them with ether.
Pull the plugs and index them to check they all face the right way.
Swap to a new GM EFI module.

 

Yes, the after,market stuff all crosses over on the interchange lists, but as we know, OEM and aftermarket parts are not always exactly the same.

I was having trouble wit mine and put in a couple different aftermarket modules and it made no difference to the poor running. I put in a genuine GM one I got at the junk yard and the problems all vanished.

I've heard of that from more than one V12 owner that new aftermarket modules behaved no differently than a failed GM module and a new GM module cured things.

 

it wouldn't be the first time that I'd seen similar behavior with brand new parts.

Do Chevy dealers carry these GM parts? All I can find are others.

Am I correct that you're suggesting part# GM 10482820? D1906 is an AC Delco part as far as I understand.

 

Im', not sure if a Chevy dealer will still carry it. They might. The GM and AC Delco part numbers lead to the same part, either will work. Last time I saw one on Amazon at a good price.

 

Ok. So today I torqued the intake manifold and throttle bodies to spec they were a bit under but not loose. Cleaned the TBs spotlessly clean. Used my phone camera to get right up in it to check. I rechecked every vacuum connection and blew out the upper and lower vac ports on the TBs.

I reassembled everything and tested . Same behavior as before with the misses.

I pulled the capstan, and cap and pulled all the plugs. All clean, dry, tan with one of B banks having a bit of a powdery finish.

I marked them all and checked for any that would be blocking the spark with their electrodes. A5 and B3 both were worst-case alignment and so i tried swapping them and they both aligned great in their new homes.

Reassemble. Reset timing since I have to change it to pull A4. My vacuum advance is right up against it and I hve to advance the timing to get it out. Is that normal? I looked at the dizzy to understand how it can be adjusted. As far I I can tell any adjustment changes the timing. Can I rotate the housing and reset the timing without cranking it right back to its current position?

Anyway I reset the timing by memory. And tested. Same behavior with the missing.

Test drive proved as satisfying as I could have hoped for! A quick blast around the block and I decided I needed to check it more thoroughly. I hit a nearby road with great curves, long stretches and no side roads and was blown away how much improvement I felt!

Awesome improvement! Now, if I could just solve that missing.

I called dealerships and all the parts store chains. GM 1048280 is superseded by GM part number 19180771. NLA from Chevrolet dealers. I'll order one online.

 

logic eventually led me to replace the EFI module in the amp with the original OEM part. I had swapped it long ago chasing some problem down.

It still runs great. It still misses at warm idle. The module is not the cause.

Any more suggestions?

 

Inside that amp is a condensor/noise suppressor, screwed in one corner with a pigtail to 1 +ve terminal of the module.

Remove it, it is redundant, and age now has them leak to earth and usually KILL the ignition system, but I have had them do other strange things.

That distributor can be raised out of the hole, and the gear rotated ONE tooth, so the vac unit is more pointing at the throttle pedastal. Fiddly, a tad frustrating, but about 1 hour should see it done,

When done, try something, back off the timing 2deg to retard.

Also, have you trimmed the ECU fuel pot, it may need a tickle.

 

I had already deleted that condenser.

Thank you for the info on resetting the dizzy orientation! I definitely want to rotate mine but I think I'll wait until I solve my current puzzle so I don't confuse things.

I haven't adjusted the ECU pot. I read someones site about it and I swear they made the creation of a adapter to turn the adjuster sound so overwhelming I still have a subconscious UH-UH every time I think about it.

I found a .doc file that I think you shared with me. No attribution, but your style.
And I feel like now I can overcome my subconscious fears and turn the clicky dial.

I should get her fully warm to adjust I assume?

 

A few thoughts:
Your original post said "Two crispy areas under ( deleted ) A/C compressor."




Do we take this to mean that part of the loom might not be 100%? If so, stripping and re-insulating the individual wires in this part of the loom is a good plan.
I am intrigued by your point that until warm it never misses, can anyone suggest what changes that could cause this symptom when the engine is warmed up?
Have you cleaned the plug and connector socket for Lucas silver resistor box?
Also try connecting a single vac tube directly from the spigot under the A bank throttle body to the vac capsule, and blank off everything else in that cat's cradle of tubes and see what happens.
A longish shot, nut it might be worth changing all the plugs and buying one new HT ignition lead and swapping each HT lead cylinder by cylinder for the new one, then the same for the coil to dizzy lead, and see if the stumble goes when a particular lead is changed.
Greg

 

Well, I haven't replaced the injection loom so it's definitely questionable. I cannot produce any engine response by roughly manipulating the crispy areas. The exterior of the areas are hardened tape from what I can tell. They are stiff. I have not cut them open yet, so I cannot vouch for the insulation on the wires within.

Resistors connectors are shiny and clean.

Vacuum advance setup is modified on my car. As pictured below...


modified vacuum advance

I will swap to direct connection to lower TB vac spigot and test.

Really doubt I have a bad wire, but a plug issue is still possible. I DID NOT swap plugs yesterday. I did check and re-gap all 12 existing plugs. The new set were all made such that their electrodes are absolutely consistently oriented relative to the threads. 6 of them would have been shrouded badly. I'm going to order a new set and see what I get.

I'll get a new lead to test with anyway. At this point I'm determined to get her steady.

I did adjust the ECU idle balance. Much better exhaust smell in my opinion. But the miss persists.

 

When did this symptom start to appear, by the way? Also do you have cats? I know sod-all about them, but maybe the oxygen sensors could be going home, they certainly change the fuelling.
Also, if you get a mechanic's stethoscope, you just might be able to pinpoint the bank and the cylinder. Love the
"Felix, qui potuit rerum cognscere causas.
Happy is he who can discover the cause of things" by the way!

 

Ok, ever since I have owned the car she's been variable. Sometimes timid and noisy sometimes squeeling around a corner with way more power than I expect.

But she has always missed at idle.

I've done so much to Jessie that I can't relate it all, many of her early issues have been fixed. All from a toolbox in the trunk and a mobile phone and you fine folks. I'm closing in on a really solid tune now.

She is running better than ever before. While this last round of refinements hasn't resolved the missing it certainly has tightened things up so much that I could easily just live with things as they are now! Except I can't.

More late breaking info that I should have included earlier. Sorry, I've tried to include every pertinent fact but there are so many.

Deleted cats after a meltdown caused by a bad rotor. On a Lucas!

O2 sensors are mysterious and therefore I suspected them long ago. I suppose they are more replace and see result than testable.

I have a good mechanics stereoscope. I suppose it might be discernible what group of three the missing cylinder is in if I listen the the down pipes. It's not exactly stereo headphones everything is noise unless you're right on the source.

I have suspicions about the coils. No reason to. Nice spark. I want to replace them both with the single. I saw a Jag part on eBay. Cheep. Shoulda stuffed it in my stocking. Maybe it's still there.

So, replace O2 sensors, get the single coil, see if she likes them?

Footnote: on a whim I popped the NAPA Eichlin module in. Jessie absolutely would not run on it. Choking down every 5-7 seconds. So I have to agree, all EFI modules are not created equal.

I've uploaded a sound clip o the exhaust. Recorded from 6" to the side of the right tip. https://clyp.it/4vqii2v1

 

That would annoy anyone! If you are sure the exhaust from manifold to tips is tight and gasproof, then I think that a one hour blast at high revs with lots of 0 to 60 blast would be worth a try too. It could just be crud on a valve.
Greg