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So most likely the orifice isn't tighten down as far as it can be. I know you say that it is, but I have been to a number of houses that say the same thing and the fact is that even 1/8 of a turn can be the difference between being converted to LP and NG. If you haven't tightened it down to the point that it stripped out, then you still have more room to keep trying. The only other than that could have caused the issue is the pressure valve which you replaced which means that your orifice is just not tightened enough or the orifice needs to be replaced because it's not working properly so it wouldn't matter if it stripped out or not anyways.
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