I used to have the same problem on my 250 and 300 but much less so on my 200 due to less power. What's happening is the trials tires can flex by more than an inch (I've actually got pictures of this somewhere) so those heavier tubes that don't stretch as much as the thinner ones will jerk the stem out when they move with the tire. I had absolutely no luck using extra baby powder, rim locks (tried two), or wd40. My problem completely stopped when I went back to a standard weight tube. - Ken
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