RE: Midrange issue - glow plug?
Daggets,
I stand behind my words...
It is true that for some carburettors, it is next to impossible to tune for a proper mixture, over the entire RPM range of the engine...
Even several carburettors of very prominent companies are so effected; like the one of the OS.61FX, tested in May 2003 by MAN.
The older MVVS #3216/3219 carburettors are similarly affected.
When set for a good idle, the OS carburettor was diagnosed as being overly rich in the mid-range.
I diagnosed the older MVVS units as such.
It is possible to fine-tune, by 're-curving' the low-speed needle's taper.
Although not many modelers would dare that feat, I consider it to be the right solution; preferable in the long-run to using a hotter glow-plug and suffering all its consequences.