So any form of restriction before the jet richens the mixture ... ?
Following that it would seem that layers of nylon stocking or a filter over a model engine Venturi acts as a choke and by default richens the mixture.
Chris.
Kind of a different take on what Jim Hoffman said:
If you get your engine all adjusted without a filter and then put a filter on -- yes. But you generally tweak the needle after such a mod, so things work out OK.
... and any form of restriction after the jet leans it out ...
In theory, no, because more restriction after the venturi means less airflow, means less gas pulled from the bowl. In practice -- have you ever rebuilt a carburetor? Get one apart, or look at a diagram of the main jets, the auxilliary jets, the power pump, etc. There's a
lot going on in an automotive carburetor that makes sure that it does a good job of delivering the correct mixture at any given time, and even then a carburetor doesn't do nearly as good as a good electronic fuel injection at calculating
just exactly the right amount of gas to put into the airstream for any condition.
Simple carburetors for lawn mowers or for model airplane engines get by with simpler arrangements because, first, you just run the thing a bit rich, and, second, at least for much of control line usage you're not changing the engine speed much.