Paul,


Most engines, when the low-speed needle is set too rich, will spit fuel from the carburettor and emit more smoke and raw fuel from the exhaust, on throttle advance... During idling, the excess fuel will accumulate in the crankcase and will be fed into the cylinder when the throttle is advanced, resulting in too rich a mixture.
Some of the excess fuel will be spat out through the carburettor.

...But some engines don't know this rule, it seems... at least as for the fuel spitting.

In those engines, you must adjust in small increments leaner and see if the transition becomes better and smoother, or worse and more hesitant. If it gets better, it was too rich and vice-versa.

Each time, after making an adjustment, gradually open the throttle fully, to clean out the engine and allow it to idle for at least 20 seconds, before advancing the throttle rapidly (as fast as your throttle servo will) and checking the transition.

Too lean a mixture will result, in virtually all engines, in a hesitant transition, which the engine clearly 'stumbles' and chokes on the way up, it momentarily becomes less audible, due to a relative fuel starvation.

Adjusting that is done by gradually richening the low end.


Opening the throttle to check transition must always be done at 'servo-speed'. I.e. do not smack the throttle open in a millisecond. Even a perfectly adjusted engine is not immune to stumbling, if the throttle is opened too fast.