RE: Midrange issue - glow plug?
Crash,
Leaning the mixture (low-speed) would have had the same effect... as a 'hotter than needed' plug.
Many, many modelers; even some experienced ones, are reluctant to touch the low-speed needle...
They seem to think it is like the tamper-proof low-speed needle in carburettor equipped, emission-controlled, late '70s and early '80s cars.
The OS A3/#6 glow-plug is way too hot for any (non-Evolution) engine larger than a .40 (even many .28s); and certainly for your 1.20.
Set the mixture much leaner to comply with a cold plug (OS A5/#10, Enya #5/6, Rossi R6/R7, Etc.).
A hotter plug is just a 'walk around'; not a solution.
Fuel consumption of your engine would be much higher than it should be.