Rear wheel locking up twice - that sounds like the piston seizeing and then freeing off, possibly caused by a weak mixture. My advice would be to get the cylinders of ASAP to see what has been happening.

Remember you must identify the reason for the fault, and rectify that.
If you just repair the fault (new piston for example), the same thing will likely happen again.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30