So I mounted this MDS 40 carb on the engine, not the best fit given the spigot is 1 mm smaller but it seemed snug enough and I don't think the O ring was leaking because you could shut if off when the throttle was closed. It possibly runs slightly more predictably than the GMS carb but really shows the same problems with stumbling rich mid range. Your point about loading the engine is right and it probably isn't helping running it in the heli with just simulated head loading. However, I think the problem is it's over loaded not under when the flybar paddles are turned flat on to the flow and the engine struggles to turn this load at a speed that allows the right mixture at that throttle opening, too much fuel opening not enough air. It runs better with the paddles as they should be when it's underloaded but still stumbles in the mid range regardless.

Which is why I now think the main problem is the exhaust which is a custom fit for the Heli. It's a tuned pipe and seems to strangle the engine causing a rich mixture until it comes on song at which point it clears the rich mixture and screams. Pinch the fuel tubing to lean it up as it's stumbling and stuttering on the point of stopping and it suddenly picks up and screams quite happily even though you've changed no settings other than clear the fuel loaded in the crank. I think this explains why both carbs show similar problems even though they are completely different designs. The GMS is a full width spray bar design whereas the MDS is half width like the engines in this thread. Funnily enough the LSN needle setting ended up with the end of the needle just flush with the end of the spray bar when the barrel was wide open. Funnier still is the fact the barrel rotates beyond the full open position a few degrees which moves the needle fully out of the spraybar providing improved fuel flow. Don't think this is any kind of design feature, more to do with MDS quality I suspect.

I think there's still some scope to tweak the needles on this MDS carb, didn't really get the HSN sorted and think it's still too rich so there's hope yet. And it would be interesting to remove the tuned pipe and fit something else ... not easy given the way the silencer needs to fit in the Heli frame.