0.5 mm should not make much difference.
A friend - 65Sprint, asked me to drive his car and tell him what I thought about it. It had a flat spot around 4000 rpm. He thought that it was lean and he was using larger than normal main jets. I lent him my wideband AFR and he found that it was too rich. He was then able to eliminate the flat spot by installing smaller jets. Even experienced drivers can find it hard to distinguish between an overly rich and an overly lean condition.
What size venturis (chokes) are in your carbs?
76 Suzuki GT500
2011 Jaguar XKR
A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke