Yeah, you normally get pinging at lower RPMs in a two stroke, though not idle, and while under a bit of load (not when coasting). You might find it difficult to identify pinging above all the usual rattles and generally hellish noises that your engine makes, but once your ear is attuned to it, it's quite distinctive.
It sort of sounds like there's a ball bearing stuck in the cylinder which is bouncing off the piston and head. It's a rapid-fire rattle. It will tend to come and go with engine speed and throttle position, depending on how severe the condition is.
If your head has been machined to match the new bore, you shouldn't have any problems, but if it hasn't and you suspect your engine is pinging, you definitely want to avoid it. Pinging is very destructive if it goes unchecked. Your bearings will get hammered. You can try running higher octane fuel and going slightly richer (drop the needle a notch). I use Shell Optimax, which helps, both because it richens the mix and raises the octane -- but my head hasn't been machined, so I do get a bit of pinging still. I avoid it by changing gears or throttle position (not ideal).
Someone passed along their specs for head machining at one stage. I can dig these up if you need them.
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