I’ve had the same problem with multiple sets of wheel. I clean them with Simple Green to get most of the crud off, then spray some Brake Clean on a rag and scrubbed both sides of the wheel lip.

I know you said you confirmed that they are not leaking from the bead locks, but it’s a good idea to change this seals. I changed them last year and they were already cracked and dry rotted again at the start of this season. Also a good idea to run tap & die kit through the wheels and bead lock screws. Hopefully that helps.

Common problem with automotive wheels, too.    Tire shops have a black goo that seals tires to the rims, maybe you can bum some from the local tire shop.

John

This is a very common problem with 6″ super kart wheels and tires…. go to your local auto parts store and buy some bead sealant.  Clean the bead area thoroughly, and after you mount your tires but before you inflate them to set them, apply a thin film around the bead where the tire meets the rim and your leaks will stop.