Main jet tuning... ...in France

What size engine and what altitude?and what cam?
I'm guessing your pretty close to sealevel, and since you didn't say 340 or 360, I'm guessing 318. And since you didn't brag on the 268* in it, I'm guessing cam is smaller.

60 to 64 is pretty small, even for a teener, with just a small cam.
Could it maybe be rich for another reason? like a too-early PV calibration,or a too-high fuel level,or an overly rich low-speed circuit,engine sucking hot-underhood air,or a poor timing curve, or engine running hot, or something else? Like headers over-scavenging on a big overlap cam,or an AFR gauge installed in a bad spot. Just asking.....