Where do you have the timing set? Is it at factory setting or more advanced? Those plugs protruding like they do into the combustion chamber could cause the engine to need less advance? Measure where it is and kick it retarded some. Is the ignition a CDI or points? I would use the NGK BP8H-N-10 plugs per the OEM recommendation, they are a great plug and similar to the NLA Champion L61Y which is a great plug for high speed running.
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