AWB 85-024 Issue 1 - points to exhaust and guide as the area of damage and wear.
This sounds like too much heat to me and lends strength to the idea that the fuel doesn't have enough lead in it. An often misunderstood area of spark ignition combustion is how lead works to prevent / reduce knocking. People think it slows the burn rate down. Actually it slows down the start of burning after the ignition spark.(Ignition lag) Once the burn gets going it burns at the same rate as unleaded fuel.(Think of starting a damp campfire and a dry one. The damp one is slow to start but once going properly it burns the same as the dry one) This reluctance to start burning delays the start of auto ignition we call knocking.
If an engine is timed for fuel with a particular amount of lead and then a new fuel with less lead is used, then the timing will be wrong. The quicker ignition of the low lead fuel will have the same effect as advancing the timing. Advancing the timing means peak cylinder pressure occurs earlier and transfers excessive heat and pressure to the cylinder,piston and valves. If it goes on long enough it will do internal damage.
Some engines are more forgiving than others when it comes to timing range but a hard working one with barely adequate cooling probably not so much.
ignition lag. [ig′nish·ən ‚
lag] (mechanical engineering) In the internal combustion engine, the time interval between the passage of the spark and the inflammation of the air-fuel mixture. Also known as
ignition delay.