What plug are you using? You say it looks fine, but how does it really look? Dark brown, tan, white?
What octane fuel, what brand of oil and what mixture ratio?
How's the underside of the piston look? I learned from blowing up pistons at the races to check the underside for hot spots. Your plug may look good, while your piston is getting torched.
How's the transfer ports? Any extra casting chunks or debris?
Did you try sanding the head on glass? Use a marker around the mating surface to color it, then sand it lightly to see if it's warped. if there's color left in spots, it's not flat. check the jug, too.
Even a pinhole leak in the manifold can expand or shrink when heated.
i know you said you swapped parts back and forth but somethings gotta be causing an airleak.
The tiniest of leaks could be sucking in enough air to overheat it and/or cause it to deisel.
What octane fuel, what brand of oil and what mixture ratio?
How's the underside of the piston look? I learned from blowing up pistons at the races to check the underside for hot spots. Your plug may look good, while your piston is getting torched.
How's the transfer ports? Any extra casting chunks or debris?
Did you try sanding the head on glass? Use a marker around the mating surface to color it, then sand it lightly to see if it's warped. if there's color left in spots, it's not flat. check the jug, too.
Even a pinhole leak in the manifold can expand or shrink when heated.
i know you said you swapped parts back and forth but somethings gotta be causing an airleak.
The tiniest of leaks could be sucking in enough air to overheat it and/or cause it to deisel.