That is not a problem on Tomos mopeds with cast wheels, they won't cut the valve stem, and the real bike shop puncture resistant tubes are far better quality than what are sold as "genuine moped tubes". I don't recommend using them on mopeds with steel wheels, for the reason you mentioned, and I think the 20x2.125 tubes may be just a little small for 2.50 tires. I have over 16,000 miles on Tomos mopeds, using those tubes, with absolutely no problems, except a few flats, but way less flats than with stock tubes. Peace. Jerry.