Pretty much everything.

I had the DUI ignition system in place when this started, so I was wondering if maybe it was an issue with the coil, pickup module, ignition module, etc. I tried replacing those, and nothing.
Next, I replaced the plugs and wires.
Finally, I pulled out the DUI and reinstalled the DuraSpark II ignition system.

So, about the only thing that remains of the ignition system is the 12v wire power source.


I haven't done a compression test recently, but last I checked it (which was before I installed the EFI system), it was around 175 in all cylinders, with only about a 2% variance.
I do need to check it again.

 

 

If I've changed out the ignition system, what do you suppose that'd tell me?
How would both ignition systems have the same issue?

I mean those questions legitimately, as I don't know much about ignition system scopes.

 

Did you change out the O2 sensor when you changed to the replacement EFI system?
It would be good to know if the compression is still the same.

 

Yes, I switched out the O2 sensor, just in case.

And yes, I have a starter relay for the EFI and ignition so that they get direct 12v battery power (with a key on switch). The large "starter" sized post goes to the ignition, and the smaller auxiliary post goes to the EFI. I originally had them on the same post and was wondering if maybe they were getting interference from each other, so I separated them (unfortunately no difference).

 

 

What is the spark plug gap?
I imagine that you also changed spark plugs?

 

That makes a lot of sense. I'm definitely all for more diagnosis on this.
I'll call around and see what I can find. It'd be great if it turned up something. I just need something I can do that will address this. I can't seem to pinpoint it to any one thing, so maybe it's time to scope it and get some diagnosis that's beyond my skill (tool) set.

I think right now I'm running the stock gap of 0.044". And yes, the spark plugs were one of the first things I replaced.

 

I'm all for diagnosis equipment.
Meanwhile try setting the plug gap to .035" and see if there is any difference in the manifold vacuum level at failure.

 

 

Justin, since this seems to be when the vacuum is very low and worse with low RPMs and being placed under a load [accelerating], is it possible that the incoming charge velocity is so low that fuel is falling out of the mixture and condensing on the intake manifold?

 

I'll give that a try to see if it makes a difference. The manifold vacuum is usually nearing 0 when I have an issue. Would you think that would change? Obviously, less than 0 and the issue would be gone.

What would I be looking for, a drop in voltage?

I've been thinking this one over since you mentioned it.

I can think of only a few scenarios that are causing the issue.

A:
More fuel is being supplied than necessary.
Although, being overly rich seems to be a symptom, not a cause.
I've had the mixture be correct, and still had the engine stutter.

B:
The ignition isn't sparking correctly.
This would cause an incomplete burn, which would leave unburnt fuel in the cylinder.
This is still a possibility, but seems like it would have been ruled out when I swapped ignition systems. But who knows, maybe something in the ignition is still lurking.

C:
The fuel isn't atomizing properly.
This is the only case I can think of where low fuel pressure/weak fuel pump would actually cause a rich condition. The spray is weak, so you get droplets instead of mist.
However, I've had many, many times where the lbs/hr were much higher than when I was having stuttering, and it was strong and crisp. If the pump can supply fuel when it's requesting 100 lb/hr at 4500 RPMs, it can supply fuel when it's requesting 40 lb/hr at 1800 RPMs.

The other reason for poor fuel atomization may be a cold intake? I did NOT add the underside intake heat to this setup, like with the carb setup.
But doesn't that usually affect it off-idle? Even though there's low vacuum, air velocity should be quite high since at 0 vacuum, you have 50 cubes of air screaming through there, as opposed to much less at lower throttle openings. There's no vacuum being produced, but a massive amount of air is moving through (the most the engine can handle), which I would think would create a lot of velocity. Am I wrong in my thinking?

 

Are the EFI and ignition on the same power circuit? I've read a little bit on the Holley EFI and it seems like signal interference is a common source of issues.

Do you have a condenser (aka capacitor) in the ignition circuit somewhere? My 95 has one that sits near the coil. I can't remember if my 86 had one though.

Here's some info on ignition wires vs. EMI: https://www.magnecor.com/magnecor1/truth.htm

 

I had thought about that exact issue in particular, and was really hoping that was it.

Originally, I had the 12v source for both the ignition and the EFI unit on the same post on a relay. I was wondering if maybe their signals were getting crossed, so I added a second solenoid that is strictly for the ignition and EFI, and each one now has its own dedicated post. (The solenoid I have has a large post and small post 12v source).

I don't know if there's a condenser/capacitor attached. IIRC, the DuraSpark II has it inside the ignition module. I know the DUI unit didn't require one and just needs a 12v single wire.

 

Maybe try running efi on a dedicated feed. You could use the existing post to trigger another relay powered straight off the battery.

I'm assuming you have a good ground to the engine? Somebody said have like a 2/0 ground when using the efi.

Just a thought, but maybe EMI is only a problem at some rpm range based on actual physical characteristics. Like a wire could be of a certain length that it's an effective antenna for some range of RPM / ignition events frequency.

On my 95, there is a certain bundle of wires going to the ignition module that is wrapped in foil and grounded in the harness. There was actually a TSB related to said wires.