Constant Misfire V12

I have an SNGB ignition conversion kit (circa 2017) and I have a constant misfire. Whether hot or cold. Behavior doesn’t change. Not very noticeable above 1500rpm but I suspect its still there. Its enough to cause idle to swing 100rpm continuously and its pronounced enough that the whole car shakes at idle. Its not rhythmic. Its like a continuous but random duty cycle misfire.

Car drives pretty nicely otherwise. Makes good power even with the misfire. Spark plugs look textbook.

The distributor cap looked a bit tired inside so I just fit a new genuine cap, rotor and gasket and it made absolutely no difference.

The car also has custom fuel injection running from megasquirt so I have good control of fuel mixture and AFR. But the misfire was there even with carbs.

I don’t drive the car much so I can’t remember when it started happening or if it always happened. I fit the SNGB kit soon after buying the car thinking I had an ignition problem but it turned out to be the carbs at that time.

I would like to try another ignition module. I know its a GM HEI system from the 70s right? How do I wire a stock GM module in place of the SNGB Opus looking box which has 2 in and 3 out I think.

How do I go about troubleshooting further in general?

I do not know anything about the V12 but if you have determined it has GM type HEI here is a trouble shooting tree from my 1975 GM manual…

68 E-type FHC

This sounds line something @Jeff_Schroeder could handle


The SNG amplifier is an improvement over OPUS but is also susceptible to heat related failure. I’m no expert on the internals but have read that the module inside is equivalent to the GM HEI module.

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I am not sure it sounds like you are convinced exactly where the problem is. If not, have you checked all the basics? Compression, valves, intake leaks, plug wires, pulling individual plug wires (if okay on the V12) narrowing down to a particular cylinder? Etc.

Compression was fairly consistent when measured. No single cylinder outlier.

T̶h̶e̶ ̶e̶a̶r̶l̶i̶e̶r̶ ̶O̶p̶u̶s̶ ̶s̶y̶s̶t̶e̶m̶ ̶w̶a̶s̶ ̶m̶e̶a̶n̶t̶ ̶t̶o̶ ̶u̶s̶e̶ ̶0̶.̶0̶3̶5̶"̶ ̶s̶p̶a̶r̶k̶ ̶p̶l̶u̶g̶ ̶g̶a̶p̶s̶ ̶a̶n̶d̶ ̶t̶h̶e̶ ̶l̶a̶t̶e̶r̶ ̶h̶i̶g̶h̶e̶r̶ ̶c̶o̶m̶p̶r̶e̶s̶s̶i̶o̶n̶ ̶C̶E̶I̶ ̶c̶a̶r̶s̶ ̶u̶s̶e̶d̶ ̶0̶.̶0̶2̶5̶"̶. What gap are you supposed to use if using a CEI system (SNG Barret upgrade) on a lower compression V12? There isn’t any guidance on this from the SNG manual.

EDIT: The Jag manual for S3 E-Type says 0.025". See attachment.

I can’t be 100% sure its happening in the same cylinder each time. Its not as bad as removing an ignition lead from a cylinder so I don’t think I can narrow it down to a cylinder by pulling spark plug leads.

You might try using the smaller plug gap to see the impact.

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I think mine are all gapped at 0.035". Thinking of bringing them down to 0.025" and see what happens. But it isn’t intuitive. Usually you have more ignition energy at idle than at higher engine speed and I have no noticeable issue at high engine speed or high engine load. But I’m at my wit’s end so I’m willing to try it.

Could you try checking at the exhaust to at least see if the misfire is consistent to one bank or the other?
I am not sure how the intake manifolds are interconnected but might it help to try a vacuum gauge to see where the misfire is?

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So I just tried it with the plugs all gapped at 0.025". Initially it felt smoother. I would get like 4 or 5 seconds of no missing and then it would misfire for several seconds and then smooth again. But somewhat random duty cycle. By the time the car got to temp, it didn’t feel any better. The misfire became constant like it was before.

Could I wire in a new standard GM HEI module and coil per standard GM wiring instead of the SNG Barratt module, coil and ballast resistor? Would it work with the SNG Barratt distributor?
Screenshot 2023-08-08 at 21-44-58 gm hei distributor and coil wiring diagram - Yahoo Image Search Results Automotive repair Truck repair Automotive mechanic

Using the 1.2ohm coil that came with the SNG kit, I bypassed the ballast resistor and module and fit a standard 4 pin GM HEI module. This resulted in lower idle but otherwise still had the similar misfire characteristic.

I also checked the alternator diode, which was good and measured 11mV of AC at the battery. Which I think is acceptable right?

Only things left to test is the pickup itself and spark plug leads

You caught my eye with “…spark plug leads.” I had a similar issue with my V12, and the problem did in fact turn out to be old, faulty spark plug leads. The problem became very evident on a moist, dark night, when I could raise the bonnet and see blue sparks jumping around everywhere! Another suggestion… don’t assume that each spark plug lead is correctly placed. Trace each one (twice) to make sure it is going in the correct firing order (don’t ask me how I know).
Best of luck,

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This is the signal from the pick up from the distributor before it gets to the amp. It hits a peak +ve of around 2v for about 5 cylinders and then 1.5v for another 5. The amplitude seems to oscillate like this very evenly.

This image was taken while cranking. Is this what a CEI distributor signal should look like?

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The voltage of the pickup signal is almost irrelevant.

What is highly relevant, is whether the pickup POLARITY to the ignition module is correct!

Incorrect polarity can still have a running engine but then the timing as viewed with a timing light is likely to be less stable than when the polarity is correct.

Have you tried swopping the two pickup wires around and then setting timing correctly (vacuum line to distributor disconnected and blocked)?

In a fit of rage I ripped out the distributor and threw 12x ignition coils on it. Still slight misfire, although a little more subtle now. Need to fully rule out electrical noise so I will try scope all the signals next.

The non rhythmic nature of the misfire is very hard to troubleshoot.

How are you controlling these coils? Just curious…

Are you SURE that those injectors are all good (clean, have similar flow rates AT SMALL INJECTOR openings)?? At idle, the injector pulse width is very small, typically just a little more than the time time it takes to open the injector (opening time). If there is a large enough difference in flow rates between the injectors when a small pulse is being applied to it, it could mean a huge difference in fuelling between cylinders, at idle (and basically very light throttle at any revs). This could then result in misfires. The higher the rated flow rate of the injectors, the more pronounced this effect becomes.

I think I will buy the injector test equipment and flow test them but I suspect the misfire is something unrelated at this point.

The coils are controlled by 2x 3 channel bosch ignition modules that came from a 1990s Volvo. I have attached the datasheet for those interested.
Data Sheet_68570379_Ignition_Module_IM_3.2.pdf (571.0 KB)

I have gone through almost every possibly combination of ignition and fueling options and the misfire just kept following.

  1. Stromberg + Opus

  2. Stromberg + SNGB upgrade/ Lucas CEI
    Replaced Opus with SNGB upgrade thinking I had a classic Opus hot failure. Turned out to be a problem in the choke metering discs of the carbs. And I still remember and very much appreciate your help Philip at the time. I was able to get the car running thanks to the chokes you sent me.

  3. SU + Lucas CEI
    This combination took forever to tune. I don’t recommend the SU HIF44 kit to anyone. It isn’t worth the time and effort. If you want my kit, I’ll sell it to you.

  4. SU + GM HEI
    This was very short lived just to see how it would run. No difference to above

  5. Batch fire fuel injection + Lucas CEI
    This was a really nice upgrade. Improved throttle response was awesome. It took less than 2 days to get it running better than the SUs ever did even with months of tuning (4x wideband O2 sensors, different needles and springs, lots of data logging)

  6. Batch fire fuel injection + wasted spark ignition (12x coil on plugs)
    I haven’t tuned the ignition table yet so it doesn’t run as well as #5 but I suspect it should if I dialed in the tables. If anyone has a 5.3L ignition table to share I would appreciate it.

I am now thinking I have some sort of valve problem where it sometimes seats and seals well and sometimes it doesn’t. Or I have some sort of electrical noise problem that is able to disrupt all these different type of ignition systems.

I have no visible exhaust smoke or obvious crankcase blowby.

I am somewhat mentally defeated at the moment so I will move onto other projects and revisit the E-type next year. I will pull the engine and start again by rebuilding a 6L and building upon that. The 5.3L in it now has unknown rebuild history but always ran “okayish” so I just left it. Should have just pulled it and started fresh all those years ago.

Wow! All that seems to only leave something mechanical deeper in the engine… I share your pain Vikram.

I should have a V12 map somewhere. I’ld be surprised if I had not posted it here somewhere.

I also say wow!
I also do not understand what is going on, and I am sure you are only providing the highlights of this story. It seems to me, rather than “troubleshoot” this problem, you have continually introduced more unknowns to the point the situation is very confused. Strombergs to SU to fuel injection. several different ignition systems. And now you are ready to rebuild the engine.

Have you done some of the things suggested above? I see a new cap, what about rotor? What have you done to confirm no vacuum leaks? Plug wires. Has a vacuum gauge been installed to check for sticky valves and other issues, etc.

If the engine is already in such shape it needs a rebuild, fine. But if the rebuild is to get rid of a miss, not the way I would go.

Whatever you decide, good luck.

All the basics were checked and consumables replaced. Entire distributor assembly, rotor, cap, wires, plugs. Compression test was done. Dry 115psi to 135psi. Not great but not terrible either in my opinion. No obvious vacuum leaks and especially on the carb+distributor setup, any vacuum leaks wouldn’t result in a misfire.

Not sure how a vacuum gauge will help diagnose randomly sticking valves with 12 cylinders. Haven’t noticed any fluctuations to vacuum. Vacuum is a very important signal for the EFI and would have seen it show up there if it was a problem. Will pay closer attention to it again.

I will try to record the misfire into something more visual to share with you all but in the meantime here is my ascii art of how it feels like to me when in the car.


Where every dash - is a second of smooth running and every x is an occurrence of slight misfire. It really feels very random duty cycle. When I got the car running for the first time with the CoP setup, it ran for about 10s without a single misfire and for a moment I thought it was cured but then the misfires came back. No different hot or cold.