Just a thought........The 91 RS has a 12V battery pack, which needs to be fully charged before running the bike. There is no alternator charging circuit from the engine rotor. When running under high load / high revs with a flat battery, you can sometimes expereince what feels like a misfire.

Also, is he using a quick-release fuel line connector? They come in different bore sizes, sometimes the smaller ones don't flow enough fuel at maximum engine speed and it can starve the carbs.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30