That's really weird. Open exhaust frequently causes very rich mixture at low speeds due to reversion, and needs leaner idle mix blah to clean it up.
I see 2 holes in the idle tube, I suspect the lower is just a feed (it's submerged in the bowl), but the top one supplies the idle & transition holes, and the 1st 3 holes have no other restriction (the adjuster only affects the 4th one). Increasing the idle tube top hole may do something, but definitely want to have a spare available since it's thin wall tube. If the top hole is 1/16" (not even a guess, just example), going up to .0635" (#52 drill) is +3.2%, .067" (#51) is +14.9%.
I would prefer to re-size the individual transition holes since they cover a different range, but it's more work and much harder to reverse.