+1 to inspect & re-install. Is your valve cockeyed? If a presta tube in a schrader rim is free to rotate, it can lead to a bulge near the valve since the valve stem is more likely to move around in the bigger valve hole. It's also possible your tire isn't seated properly on the rim, which is a blow-out hazard as noted above. You should probably be able to see/locate the bulge. Inspect to see if there is any damage to your tire.
Originally Posted by
dabac
I've never found any benefit from Schräder-to-Presta adapters other than that reduce the amount of stuff getting into the rim via the gap around the valve...
A presta tube in a schrader rim can start to press out of the valve hole, causing the edge of the valve hole to cut into the tube. This will often lead to a flat over time, especially at higher pressures. I know this because...
