Something else you may want to consider as a possible fault - crankcase seals. Your problem is on the rear cylinder so if it was a seal issue then look at the crankcase seal behind the Flywheel / Stator.

Before I had major cold starting problems, I noticed that the rear cylinder was running lighter, with regards to pulses out of the pipe. I did a carb vacuum check and found the rear pot carb was pulling about 30% of the front pot carb.

When riding the bike, it was foggy as hell under around 5000 rpm and just wanted to bog all the time. But it still ran very well over 8000 rpm every time.

I did a crankcase compression test and found the cases would hold zero compression or vacuum. You've done a cylinder compression test which only shows the rings are OK. You could try spraying 'carb cleaner' or 'easy start' behind the flywheel on tickover and watch for revs rising, that would tell you the seal is sucking in. But remember you really need to test the seals in both directions to be absoluetly sure.

Your problem could well be something else completely, but I wouldn't rule this out from your fault diagnoses. I stripped the carbs several times thinking it was a fuelling issue before I tested the cases.

This thread shows my problem:
http://www.nsr250.net/forums/viewtopic.php?t=7023
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30