It could be a failed crank seal on the front cylinder side. The engine is basically an air pump, so if the seal integrity is lost on one side it won't pump air as effectively / strongly as the other side.

Don't panic just yet, check the basics / obvious parts first - ignition / carbs etc. Then check the piston rings, make sure you have decent compression. If all that checks out, think about doing a crankcase pressure leak-down test.

Do also search the discussion forum for these topics, all have been covered before.

Good luck.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30