Terry, I use the o-ring, and then apply RTV as a backup. I do this on all my new engines to prevent any air leaks at the carb to crankcase, which seems to be a common problem. If there is an air-leak at this point or at the backplate, the engine will not adjust properly or shut down when the throttle is fully closed.

Britbrat, the Tower is supplied with both a front or rear needle valve option. I prefer the front NV setup too. As far as the ringed GMS versus the Tower, I've dealt with both of them and think the Tower has a much better carb, which offsets the higher cost of rebuilding the non-ringed Tower when the time comes. Both seem to put out about the same power.