[Stubby79]

We'll find out after I've done the brakes and take it for a drive...if all is well, the shims stay...if it's the same, I'll get around to it later...if it's worse, they come out ASAP.

Timing chain? Doesn't fit with my concept of the circumstances. Might explain the miss under load while in lean burn, though I'd expect such issues all the time under load. If I took my foot off the gas then punched it, I might expect it to behave that way, but the engine was already under load - throttle open, say, 20%, when I punch it. There's no engine going slack(going from pushing to being pulled then back to pushing again suddenly) to cause any slack in the chain to mess up valve timing momentarily.

My...assumption...is that there's no way for the fuel delivery to respond immediately when you punch it. There's a sudden flooding of air in to the intake, the injectors are only firing enough to do lean-burn A/F ratio, so now, for that one brief moment, the A/F ratio goes extremely lean, and can't ignite it...or there's enough air in the cylinder (and not enough fuel to cool it down) to cause the compression to ignite the fuel prematurely. Either way, you get a misfire, just for that one cylinder that's firing during that one brief moment it takes the engine/ecu to compensate and start flooding in fuel to match the amount of air going in.

It's just a logical guess. If the engine goes stupidly lean for a moment, it would certainly make the engine hiccup like that.

It's probably not an issue with a normal - non lean-burn - engine, as there would be enough fuel to fire anyway. Lean, sure, but not enough to cause anything noticeable. Plus the ECU might simply take a moment to switch fuel maps, which a non-lean burn wouldn't have to.

AND...just to add more fuel to the fire...the programming of the Insight's ECU is presumably set to maximize fuel economy...it ignores the TPS telling it that the throttle has been opened suddenly, until the MAP sensor confirms the sudden change in manifold vacuum/pressure...by then, the air is already flooding in to one of the cylinders, and it's too late to fire the injector for longer.

Anyway, if it's pre-ignition, from too much compression/too lean, in theory, higher octane fuel could fix this. Anything fresh is probably higher octane than the ancient gas that was potentially in it, but high octane would help even more. I didn't get to drive it enough to decide one way or the other before I messed around with the spark plugs yesterday. It seemed to be smoother - not perfect, but better - with the fresh, high-octane mixed in, so I didn't get annoyed enough to punch it trying to get it out of lean-burn.

I could see it misfiring in lean-burn due to too much spark plug gap at the larger A/F ratio in lean burn, made worse combined with heavy load. So the spark plug gap had to be checked. If the indexing of spark plugs affects air flow/ignition of the fuel, that could be just as important.

From what I understand, Honda used to safely/successfully ignite their lean A/F mixture by sending in a second, richer (stoich, presumably) air/fuel charge that was easier to ignite, and in turn could ignite the rest of the leaner charge in the cylinder...if you assume they use a similar technique in the insight, using the sequential injection to dump in a richer charge right before the valve closes, then indexed spark plugs could be absolutely necessary to ensure that the richer charge is ignited properly. (If it's homogeneous - yeah right - it probably wouldn't matter nearly as much)

More theories/educated guesses. Whatever. What we do know is that misfires are caused by (bad) a/f ratio, heat build up (compression/hot spots on valves), and/or not being able to fire the spark plug. I'd lean towards one of these causing the issue, regardless of the why of it. (Though good luck fixing it without knowing the why of it!)