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Well, due to almost daily rain/thunderstorms I have not driven the Reatta with any regularity for two weeks. Additionally, the town has been ripping up and repaving my street. They haven't finished yet. There is a big gully at the end of my igloo driveway where it meets the street. I filled it in temporarily with dirt this morning so that I can head to a cruise-in driving my Reatta. The contractor most likely will be pissed that there's dirt in the gully when they come to do finish work (whenever that day comes!). Suck it up contractor...do your job!

This is the circuit diagram for a four cylinder but the principle is the same. It appears the current flow is in series where one plug fires forward and the mate fires in reverse. I always have a hard time getting my head around the fact that the plug center and side electrodes are isolated from each other and the only way for current to flow is by jumping the gap, no matter which way it is going. I would guess if a hypothetical runner can jump a ten foot gap, they could also jump a five foot gap, and more easily? But what if there was a series of gaps to jump, maybe fifty or a hundred? 😖

Posting here for future knowledge: the issue was in fact the harmonic balancer. My Handiman said the old one looked like it had been “beat on with a hammer.” replacing the balancer did fix the issue. I guess the moral of the story, is if you have a weird engine vibration you can't track down, it might be the balancer. Even if you don’t have the usual sound that occurs with a bad balancer.

Are you certain the engine is running on all cylinders when that occurs? A dead cylinder can cause the engine to feel like it's vibrating. Here is something I usually do if I suspect an engine is missing. Park in a safe location. Put the transmission in drive and hold the brake really tight so the car can't move. Then start pressing the accelerator down slowly with increasing force to put a load on the engine. See if at some point the engine changes tone and starts missing or a cylinder goes dead. If that happens you might have found your vibration.

For context, this is a 1989 replacement engine in the 1988 Reatta. The balancer is apparently not original but it is unknown if other items were replaced inside the front cover. Is it possible the balance shaft is out of phase if a mistake was made doing a timing set? Unknown. This was a little snippet I found for the reason for the strobe or timing light. Strobe Light Applications for Machinery Problem Solving (automation.com)

And here’s 3 more videos that might help, technology is wonderful. 1 taken at 240 fps slo mo: https://share.icloud.com/photos/0d0UPSC-A181i_uabJDkPP8yQ 1 taken at 120 fps slo mo: https://share.icloud.com/photos/0e78C8po8bxexWWweh4mtNW1Q and another taken in normal video mode, but at 24fps: https://share.icloud.com/photos/003ljmheph2NGnYxLsw3OWDKw maybe these can help? In theory accomplishes the same thing as Ronnie’s earlier timing light suggestion?

I would check the corner supports that go over the battery and air cleaner to be sure they are both there and tight.

So, I’m still pretty new to this working on my own car thing. I’ve not ever done the procedure as you’re describing. so is performing that test the next step I need to take or am I missing an intermediate step?

'88-'90 had a slip fit on the balancer. Normally shouldn't need a puller to remove it. '91 has a tapered fit and needs a puller to get it off.

I looked at the two engines on stands, an 89 and a 90 and both have no holes in the front for puller bolts. The problem is, that doesn't necessarily mean a modern aftermarket one isn't okay. It might be as simple as consolidating part numbers? The Series 1 engine internals are very similar to the LN3 but they are not identical. I think the pistons and rods are slightly different and the crank is different in that it uses a one piece rear seal. I wouldn't think the balance factors would be wildly different, if at all, but ???? I was just looking through the parts book and the crankshaft and balancer are the same for all three years of the LN3. The L27 from 91 on does list a different crankshaft, balancer and flexplate/flywheel which is two piece?? That still doesn't mean much of anything as Padgett pointed out, painting a different color changed the part number.

I don't want to muddy the discussion but has anyone noticed some similarity in the rpm bands that show up as misfires, vibrations, poor running and other observed problems? I don't know what that means, if anything. At 1500 rpm and three power pulses per revolution, it works out to 75Hz. I have no theory if there is a harmonic that manifests itself? I have a rattling heat shield on another car that only does so in a narrow rpm band, so it got me to thinking??