Was the airbox + pipe mods already done to the bike, or did you put them on?

Have you had the carbs off for inspection (clean jets) and also check all jet sizes, against standard, and taking into account the modifications?

Tuned pipes and free (er) flowing airbox will create a leaner mixture.

If the problem you describe is developing over time, it could actually be something more serious, such as a hardening / degrading crank seal allowing air to be sucked in, thus weakening the mixture. But do the easy checks first.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30