1980 Chevrolet P20 Step-Van 8 Cyl 5.7L
my p 20 had 2 plug wires melt at the spark plug boot .
i dont know the mileage, the engine gets very hot under the engine cover , but the 2 plug wires that melted at the plug boot were right across from each other, very strange. Could it be that the exhaust manifolds are getting too hot? and if so what could be the cause. at the time this happend it was very hot outside, the engine does not overheat.
Maybe the engine is running too rich and the fuel is burning in the manifolds as you say and the manifolds are getting too hot. Check purge valve near the thermostat housing if it has a purge valve. Remove purge valve hoses and see if there is wet fuel inside. Should not be wet fuel dripping out any hoses attached to it. (It may not use a purge valve and I have no way to know if it is there)