An overly rich setup won't detonate, it'll just foul it's spark plug and not run.

Andy's right and lower compression is "safe", however, just the markings on the barrel/head are no guarantee of setup, my heads have been machined for instance as have any barrels that've been replated.

Stock squish for an NSR is around 1.2mm, you can measure this when the engine is assembled by poking a piece of solder into the plug hole (make sure you get it right to the outer edge, then turn the engine over on the kickstart and squash the solder between the piston and head, the resulting squashed end of the bit of solder is the squish band thickness measurement. 1.2mm is stock, 1mm is R barrels, SP heads and requires some careful setup to run on the road, much less is unsuitable for pump fuel.
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