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Vacuum should be very steady, like varying only +/- 1/2"Hg at idle, with reading being in the 16-22" range. If the reading flutters, like +/- 5 " or so, then I think that indicates a valve problem. I am not an expert in interpreting vacuum readings, but there should be many guides online...

I doubt, then, that this is an ignition problem. Not the plugs, coils, PCM, or ICM (if it has one). As I recall, you have swapped injectors too, with same results, problem stays primarily with 6 and 8; correct? So, I am suspicious of valve operation. I would take off valve covers and check...

Sadly, any of us will be just guessing without a schematic. If nobody here comes up with a schematic soon, consider buying a manual or subscription to Alldata. It will be cheaper and less frustrating than just throwing parts at the problem. This could even be a case of the new FP assembly...

After swapping the plug wires, is it still 6 and 8 that have the (most) misfires?

Do you have a schematic of this circuit you can post?

Have you tried it with the EGR disabled, both ports blocked-off?

Well, I read-up enough on Kioritz/Echo/Yamabiko engines to believe that is CDI, not Kettering. I better leave you be. I have no clue what all the bits and pieces of that system are, much less what typical coil resistance would be.

Take two of those "not long enough" HEI wires and splice them together end-to-end. Put a couple of layers of tape over the splice and route/tie the long wire so that the splice is several inches away from any ground. Do you see any difference in the knock-retard events in live-data now? I...

Secondary maybe 10K ohms, or more. The primary will only be an ohm or two, too low to measure accurately with a common multimeter. Does this have breaker-points; have you chased that black to find out what it connects to in the crankcase? Where is the flywheel/magnet in that picture? To me...

It is certainly a start, but there must be more pages. I don't think the blower in question is even on the page posted.

I find very little info for a "Mantis 20"; and am surprised it would have CDI. What makes you think it is CDI? Maybe post pix of the engine/ignition components. Does this coil have two windings (primary and secondary), or is there only one high-voltage winding?

We need a schematic of this circuit.

Sure, the VSS should be zero, but is it??? Does it work if you spin a wheel by hand?

Is this car for your own personal use? I would be tempted to bypass that troublesome FP circuit with a toggle-switch on the dash. Put it hidden under the dash and it will be a reliable anti-theft, too...

There are just two wires to the IAC valve, correct? I think that is a PWM system, at about 300Hz. You should be able to get a meaningful DC voltage reading across those wires. Compare what it is doing when it first starts and runs as opposed to when it begins to run rough; we may get some...

You are looking in the right area, but I don't think yours has the elbow I am talking about. Keep looking at all those rubber pieces, though. The guy above had problems with a different elbow...

No, more like this. Leave the coils in place and just extend the high-voltage wire. https://www.rockauto.com/en/moreinfo.php?pk=10474232&pt=7224&jsn=464 Just be sure to get a set with wires long enough to cross over the engine, that particular item doesn't seem to spec the length. You can...

I can understand that the stock CNP wires are way to short to swap. Consider getting cheap set of "universal" wires to do this.

Hmm... I just peeked at a picture of the coils. Looks like you have 8 individual coils, "CNP". Most (all?) of the below is wrong! There is still a hint that the CMP (cam) sensor has failed and it has defaulted to "wasted-spark". Try swapping the 1-6 and 5-8 wires. If it has defaulted to...

I have been struggling with how to help here. I do have some suggestions in mind, but let me study your posts and my reasoning a bit more. I agree the plugs look pretty good. Compression (150+) at your altitude (3900') is reasonable, but we should keep in mind that compression would be fine...