Well I can think of several reasons that this engine would have three different types of spark plugs in it. None are any good?
By saying three types, I presume that you mean three different heat ranges, not just brands?
In any case, that is an "interesting" discovery since going to EFI IR induction is because using Weber 48IDA carbs is not accurate enough for most.
Fouling plugs is one of the "notorious" fallacies that "Webers" have and this here spark plug thing is a very clear indication to ME that there are issues with the fuel injections inability to NOT foul plugs?
VERY interesting indeed!
https://www.youtube.com/watch?v=5Qf6Sv3A9zs
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