The best solution is to replace the O-ring.

That piece of silicone tube needs to be a little longer than the distance that separates the step of the needle and the valve body after the engine has been tunned.
In that way, the silicone will compress and will avoid air leakage.

For my engines, the silicone grease has been more effective; see this thread:

http://www.rcuniverse.com/forum/m_93...tm.htm#9359588

Your problem may not be valve air leaks, though.