Technically that's not quite right depending on how pernickity one is. At a
given no load idle speed, say 800 rpm, regardless of whether that's achieved
with a weak mixture and a little more throttle or a richer mixture and less
throttle the engine is developing exactly the same power. We know that as a
given because the load is the frictional loss in the engine turning at that
speed and that must be a constant by definition regardless of how the carb
was set up.

When drive is engaged the load increases by a fixed amount and the revs
drop, to say 650 rpm. If the power output is X bhp at 800 rpm regardless of
mixture it should also produce the necessary Y bhp at 650 rpm regardless of
mixture as long as that mixture stays the same.

However what happens is that as the rpm falls, the air speed through the
carb also falls and the mixture weakens further unless there is a very
sophisticated idle jetting arrangement in the carb. It's this weaker mixture
at 650 rpm that causes the engine to not produce the necessary Y bhp if the
mixture at 800 rpm was too weak to start with. In other words the mixture at
800 rpm need to be a little richer than necessary so that it becomes
correct, or at least sufficient, at 650 rpm.

That's also the reason why an overly rich mixture at 800 rpm shouldn't cause
a stall problem so readily as an overly weak one because the excess richness
self corrects as the rpm falls.

That's a rather long winded way of saying something not too different to
what you already had I guess.
--
Dave Baker