It’s been a very long since I’ve had a JR and it’s going to be awhile until there will be another in the family, but here goes.
First off, don’t be hesitant on trying different setups. If nothing else, you’ll what not to do the next time. A couple of things to try besides tire widths: Moving the seat up will result in a higher center of gravity which will result more weight on the outside rear wheel. Is your driver leaning into the turns?
Try adding more castor. That will increase the jacking effect and again get more weight on the outside rear tire. Changing your tire could help as well.
If you are struggling and desperate, ask the people you race with. If there is a local kart shop that supports younger drivers, ask them for help.
LAD Specialties customer / tony kart / rotax / kt100