I also had the same problem with a 50i and a tillitson hd232 carb and and plenty of hours. I asked for help here and got nothing. My engine mixture would change even when turning / rotating model over on the ground and was worst at about 1/2 throttle. Ian nicols at IADMODEL DESIGNS in scotland put me on the right track. He said that it was a problem with the needle seating and the weight of the diaphram and needle going from say vertically done to vertically up. In the end I fitted a few walbros from chainsaws as a test and saw an immediate improvement.I finally fitted a carb from a DA, as it was all that I could get. The problem went away until ignition packed up. The tillitson has 4 holes in the venturi and makes for better and smoother midrange, the walbro gives more of a step type of throttle response. Having proved it with various carbs even smaller bores that they all ran and ran better even if they were off a smaller engine eg 30cc. So who knows. The 3w has been replaced by a mt 57 because reeds are now worn (£20) and ignition faulty (£119) excluding postage and new engine £250. Hope this helps because the 3w is a powerful motor ie lots of torque loves large props at about 6000 6200 rpm