By needle valve retaining screws, do you mean the two small brass screws on the side of the carb where the high speed needle valve is mounted? If so, then definitely tighten those down, but before you do that you might want to remove the whole carb, look at the spraybar from the underside, and make sure that the small slit in the center of the spraybar is centered before you really tighten things down. If those two screws are loose then it's possible for the spraybar to rotate and throw your low and high speed needles out of whack. Good luck with the engine. I have one and it's really starting to grow on me. They take a little tinkering with at first, but once you get the hang of it they're really great engines.

Dave