ORIGINAL: DarZeelon
Jim,
With a good carburettor that has linear fuel metering, a two-stroke engine running at a part throttle setting, is not much different from the same engine running with an extra small carburettor.
It is true that most carburettors supplied on two-strokes, like those from current OS and most clones, cannot be adjusted for a perfect part throttle mixture and they do tend to load up in the mid-range, around 1/4-1/3 throttle, when adjusted for a perfect idle mixture.
Dave Gierke found this behavior in the .61FX, in that MAN 05/03 'engine shootout'.
Engines that have a more precise low-speed needle taper (like MVVS and Webra), or that have a means of adjusting the mid-range mixture, like on some older F3A carburettors from OS, Enya, Etc., or the current Super Tigre rotating fuel sleeve, will not suffer from a bad part throttle, after adjustment is done.
Four-stroke engines actually use the same carburettors as two-strokes, yet don't suffer from this loading-up (unless it is a YS, there is no raw fuel under the piston), so they are less sensitive to an overly rich mixture, at part throttle settings.
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Actually, it depends upon how the two-stroke engine is timed/ported as to how well it will handle partial throttle operation. Such factors as crankcase stuffing, lower piston fit to the liner, compression ratio, the amount of nitromethane being used in the fuel, glow plug heat range, etc., all play a significant role in tractability.
Smaller throat carbs keep the gas velocity high, which aids in atomization, hence keeping the fire lit. This is particularly important at partial throttle settings, especially when used with larger, higher mass, propellers, as in 3D applications.
The superior exhaust scavenging of a four-stroke engine aids in the four-stroke's ability to recover from a partial throttle setting and return to high speed operation with increased reliability (less hesitation/stumbling). Four-strokes are clearly superior to two-strokes for 3D flying because of this trait. This is especially so when higher percentages of nitromethane are used in the fuel.