RE: MDS 48 mid-range issue
Many, many carburettors on model engines, do not meter the fuel properly...
Among them are the older MVVS, many from OS, older Rossi and others.
The top-end and the idle can be set perfectly; and the transition from idle will be nearly perfect.
But if after you set the high-end and the idle perfectly, you try to run the engine in any part-throttle position; the mixture will be way too rich.
And after running at that setting; even for just a few seconds, transition to full-power; as well as down to idle, will be lousy...
It may even cause the engine to conk-out.
Enough fuel would have accumulated in the crankcase, to prevent a smooth pick-up, or drop down.
Trying to 'twiddle' the needles will get you nowhere flat...
A different setting that will make the mid-range leaner/better, will necessarily make the idle mixture too lean...
If this problem is encountered, a master can 'recurve' the low-speed needle's taper, to allow it to be closed further; leaning the mid-range, without causing the engine to lean-out at idle. Some manufacturers (MVVS) sell needle-nozzle sets to get this effect.
For anyone else, a better replacement carburettor is in order.
I can recommend those made by Perry, Super-Tigre, the new MVVS, the new Rossi and Enya.
Some will need to be made to fit the target engine.