Lancair, I had the same problem with my 50sx turns out the problem was the idle stop screw had loosened and backed out too far and was allowing the throttle barrel in the carb to vibrate a lot. When I looked down the carb with the engine running at full throttle the throttle barrel was opening and closing just enough to make the engine surge. Make sure the idle stop screw has not backed out too far. ..


Just an idea. Good luck. Bob