The teflon tape idea is a good one. But the teflon tape is very fragile, just a few turns of the needle valve causes it to fragment and break up and now you have little bits of teflon tape getting into the fuel ports and orifices on the carb.
I have used a little silicon grease or even mineral based grease on the threads and that works fairly well. But you have to be careful and keep the grease off the tapered parts of the needle valves. I usually use both a short piece of silicon tubing and a little dab of grease on the threads when I was fixing leaky needle valves before.