Yep, it needs to be under load. But, the "run flat out for 10 seconds" is more than a little bit extreme and ironically unless your bike is pretty close to spot on (or rich of course) it probably won't last that long.

But if you're confident it is OK get it up to flat out in 6th and as it just pulls the max it will (preferably up a slight hill, don't worry about the 10 seconds flat out) hit the kill switch and whip the clutch in. Get the plugs out (both of them) and have a look.
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