I was wondering.

On a carburetor, the idle circuit is variably adjustable by a needle screw, right? So, it's like a non-fixed jet? Why isn't that done with the main circuit?

That how it seems to work on older small engines, with the main adjustment on the bottom of the bowl.

I did a search and found this.

but, It looks like this for small engine/agriculture equipment. So... why not automotive engines? Why use a fixed jet that has to be changed for adjustment?

Or do I have this whole thing wrong and don't know how a carburetor works?

 

stupid question or too good of a question?

 

what kind of a carb are you talking about what you showed was not on a car maybe on a model T or A
if you have more info that would help

 

An automobiles carb will have multiple fuel circuits, each one dealing with a specific rpm or load to keep a consistent fuel mixture over a large rpm range. Small engine will have a much smaller rpm range and predictable single load making a simple carb with two fuel circuits enough to manage the demand.

When you look into auto carbs you'll find how complicated and precisely engineered they are.