Forget the silly plugs.
Clearly your ignition timing is too far advanced &/or the compression is too high for the gas you are running. I believe you mentioned the timing problem previously. Once they seize they often will keep doing it as the ally is smeared about the place & keeps picking up & needs to be removed.
Caustic soda on a damp rag attacks ally. Be careful with it though, dangerous for eyes/fingers etc.
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