Don't mean to worry you, but when my 21 had a dead crank seal (ignition side), the left side exhaust note on tickover was like a wet fart.

But that was a pretty extreme problem. Your carbs could be out of sync, power valve on rear cylinder not adjusted correctly, exhaust port / valve coked up???
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30