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I see the high carbon build-up as oil-related too, but more is to be expected at 170K. One clue is the nice clean plugs from plug change #2, although do look like iridium as opposed to the last set.
I actually can't see a chipped insulator on #1. The photo looks more like some carbon has chipped off a normally-sized insulator. For that reason, and not fearing detonation, I wouldn't hesitate to go one heat range warmer with the next set.
The driving style may have something to do with some of the sootiness. The 170k plugs shown appear actually too cold for the average combustion temps, adding to the sooty look. Those low combustion temps are typical of such a large engine when used for light duty, though.
CARBON DEPOSITS
The most likely cause of oil ingestion are the valve guides. There was a time when no engine made it much over 100k without a valve job, and most needed valve work around 80k. With better oils, better metallurgy, cooler exhaust temps, we go beyond that now. But the stems get pretty loose even as the seats still seal well. A problem on a large engine is, very little throttle moves the load along. But less throtttle means that the valves run operate in an exceptional high vacuum (manifold vacuum) almost all the time. The piston intakes suck oil (along the worn stem clearance) from the valve cover nearly constantly. The umbrella seals (under valve springs) are often uselessly loose after 50k. From wear, blowby into the crankcase has increased that the PCV may not prevent crankcase pressure. Between pressure on the oily side, and vacuum on the intake side, cylinder oil ingestion usually increases dramatically on high mile engines. New engines maintain a slight vacuum in crankcases, helping prevent this... and helping prevent oil leaks from gaskets.
Speaking of PCV, I am surprised to hear that your engine does not have this device. As far as I am aware, this type evacuation valve has been standard on all engines since the early 1960's. Prior to that, there were vent tubes aimed at the ground under the engine and vented caps on either at least one valve cover or the oil fill tube. The PCV was one of the first items to appear when pollution became a concern. Most engines have a PCV device drawing oily fumes (unburned gas, exhaust, etc) through a hose to the intake manifold from one valve cover, and one hose going from the air cleaner housing to the other valve cover to provide clean replacement air if needed, and/or to provide an over-surge path for excess crankcase pressure under high power. This air cleaner hose has the nasty habit of soaking the paper element with oily blow-by after the engine is well worn (and the PCV nearly useless at keeping up). Making sure the PCV is not clogged is an important part of reducing oil ingestion. The valve bobbit should pop to a low-pass restriction under high vacuum (for idling) and spring back to wide open under low vacuum (acceleration). It is designed to match the amount of blow-by likely present. You do not want it stuck near closed under high power, or the intake vacuum will not evacuate the crankcase.
THE IGNITION
The ignition voltage is a tricky thing, but not so bad when one intimately understands it. The first rule is that, if they fire at all, all plugs fire at the same exact voltage under the same conditions, no matter how lame the coil pak is. The maximum ignition voltage is solely determined by the minimum necessary electromagnetic force build-up (voltage) needed to ionize the air/fuel vapor and create a complete spark path. When the ionization extension is sufficient, the spark suddenly jumps the gap along the conductive ionized path (identical to lightening). Keep in mind, for the same cylinder pressure, plug and fuel mix, every repeating spark on this certain plug requires exactly the same voltage, no matter what fancy device produces it.
But there are variables. If the pressure goes up (air is an insulator), so does the need for greater voltage to force the electrons to jump. That is why low speeds at wide-open throttle can cause misfires. Scads of slow-speed available air molecules, highly compressed, are difficult to force a spark through. On the other hand when at low throttle (closed butterfly), very little air is available to the cylinder and the piston literally compresses a vacuum. Spark loves to jump such a vacuum because it is easy, vacuum being the very reason the old vacuum radio tubes conducted so well at very moderate voltage pressures.
So what about oil and carbon fouling? The thing is, all coil paks (including the antique points/coil) gradually build up a voltage as magnetic lines build and fall through the windings. Ideally, the electricity can go nowhere else until it builds critically high enough to jump the gap. But dirty, carboned plugs leak the voltage away as it tries to build, thereby lowering the available max voltage, and worse, drain some of the narrow amp pulse away (literally volume of electrons), so the squirt of electrons (spark) is anemic. The process can still most often eventually reach the same exact mandatory firing voltage, but the spark is skinny and cold, just like we can sometimes observe, when a spark fires a weak red streak instead of a fat, vibrant, healthy blue. In this same way, the same amp leakage occurs when plug wires become porous with age and allow leakage through lesser air insulation pores or even short-circuiting dampness. If leakage is bad enough, the plug will not even fire (reach "jump" potential). But, if the plug fires at all, it always, always fires with the same voltage for the same conditions. That is the #1 physics rule.
Another voltage requirement factor is the amount of fuel in the mix. Richer is better up to a point. Fuel is a good electrical conductor. When the mix is close to stoichiometric, it is a reasonably dry mix with air and fuel molecules interspersed well, and fuel particles bridge gaps in (assist) the ionized air spark path. But excess fuel may result in wetting the plug insulator, leaking voltage away like carbon deposits even as it builds. Therefore overly lean fuel means that the excess air is difficult to all ionize compared to a proper mix that conducts electricity much better. For that reason combustion chamber designers try to have the richest mix near the plug to promote firing. If this "conduction" fact were not true, gas engines could fire any mix, and control engine speed by fuel-only, exactly like diesels. Some engines (DI, Direct Injection) can do this, requiring no throttle butterfly valve at all. The efficiency of these DI gas engines closely approaches the low pumping loss of diesels.
Another variable is how sharp the edges of the spark plug nodes are. New plugs have nice sharp edges and fire much easier than worn, eroded electrodes. Sharp, as in lightening rods, not round as in antenna tips (the little ball) to prevent static discharge noise on the old AM automobile radios. Sharp plugs fire at lower voltage with far less chance of crossfire. When I was in high school, I worked at a service bay that had electrical spark plug test equipment. A frequent new-plug customer, a cool guy older than me, had a Pontiac GTO that often needed new plugs to fire reliably at rpms in drag-races. From just minutes of high combustion temps, the new sharp edges eroded quickly to a slight rounding in plugs. I was able to demo him that his old plugs could be economically cleaned and re-sharpened (filed) to fire just like new. I saved him some bucks and we became friends over it.
Wes
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