The presence of Lead on the back of inlet valves doesn't surprise me.
Gasoline has an aggregate molecular weight of 145. Tetraethyl Lead (TEL) is much heavier with a molecular weight of 323. I don't know for sure but I suspect that commercial TEL was never 100% pure and that it contained small traces of even heavier stuff liked Tetrapropyl Lead & Tetrabutyl Lead. Now while this stuff was perfectly soluble in liquid gasoline, as soon as you 'pull' it into the intake air in a carburetor, the different species in the gasoline will evaporate at different rates (especially if both the fuel and air are cold). Very light stuff like Butane will always rapidly enter into the gas phase. TEL (and particularly any heavier associated junk) will volatilise more slowly. Should any TEL still be in the liquid phase when it hits the back of a hot inlet valve, it could decompose and leave behind the type of deposit you found.
Nowerdays we don't have lead in gasoline but the same principles apply to any heavy stuff that gets into then intake air in that, relative to the fuel, it's going to be slow to evaporate (and may not evaporate at all). So oil that traverses the PCV system and the heavy additives that get added to gasoline fit this bill and are likely to play a role in deposit formation.
Gasoline has an aggregate molecular weight of 145. Tetraethyl Lead (TEL) is much heavier with a molecular weight of 323. I don't know for sure but I suspect that commercial TEL was never 100% pure and that it contained small traces of even heavier stuff liked Tetrapropyl Lead & Tetrabutyl Lead. Now while this stuff was perfectly soluble in liquid gasoline, as soon as you 'pull' it into the intake air in a carburetor, the different species in the gasoline will evaporate at different rates (especially if both the fuel and air are cold). Very light stuff like Butane will always rapidly enter into the gas phase. TEL (and particularly any heavier associated junk) will volatilise more slowly. Should any TEL still be in the liquid phase when it hits the back of a hot inlet valve, it could decompose and leave behind the type of deposit you found.
Nowerdays we don't have lead in gasoline but the same principles apply to any heavy stuff that gets into then intake air in that, relative to the fuel, it's going to be slow to evaporate (and may not evaporate at all). So oil that traverses the PCV system and the heavy additives that get added to gasoline fit this bill and are likely to play a role in deposit formation.
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