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.