I’ll share our recent experience as it seems relevant. My son’s JB WF is essentially brand new and rebuilt (twice, unfortunately) by Eric Nelson at 7th Gear. The motor is strong, but Kieran was having exactly the issue you describe: terible launches and terrible recovery from slides and sharp turns. Gearing didn’t matter and we tried several different ones. Importantly he experienced the same things when he briefly drove a loaner motor while his was being rebuilt. The only common piece between the two motors? The brand new clutch. That’s foreshadowing.
After a few events where he wasn’t improving, I decided that a track day was in order to get us both back in the swing of driving fast. It worked wonders for two reasons: we got more seat time that one day than we’d had for the previous year combined, and his clutch finally came in. My guess is that either the shoes had to be scrubbed some or the drum needed to be heat-treated as we did at the track. The result? Kieran went out and was faster than all of the JB and JA kids at Summer Nats and finished the Champ Tour paxed 13th out of 213, his best finish in a National event ever by a long shot. His launches at the Pro were great and his clutch didn’t falter all weekend. He said it was a dramatic change for the Pro and we were able to gear him all the way down to 17/65 for the Tour.
So my hypothesis, right or wrong, is that autocross doesn’t allow the clutch drum to heat up enough for a proper break-in/coating. The answer for us was a track day to allow that to happen. Alternatives might be to sand down the clutch shoes a little to get to the good material and/or rough up the inside of the drum to make material transfer or heat-treatment to more readily take place.
I was almost ready to ditch this motor and head back to the KT100 (and would have if we’d still had them around). Kieran was not having fun until this change and constantly asked about going back. Now he’s VERY happy with the motor and we’ll work on weight-balancing the kart better now that I know we’ll have it for several more years.