You,ve gotten good advice here, except I wouldn't go out as far as .038 for gap on these engines unless you have a hotter coil. You won't get a good idle.
The reason the platinums don't work well in this engine is; first the picture above says all about the first reason, second the center electrode is too thin producing a thin spark which effects idle quality. The copper core has a fatter center electrode which produces a fatter spark.
I gap copper core Bosch plugs at .034, and it works good for me with stock ignition.
Have a great day,