Main jet won't lean out

[J Witt]

I can turn my mixture control all the way in (clockwise) until it stops, but I still don't believe the engine is running lean enough (Zenith carb), and I certainly can't get it lean enough to stumble and die.

Is this the normal range? It wasn't in my Model T carbs.

I can rich it up some and it is immediately running a little less smooth, leading me to think it could be leaner. I removed the needle assembly and it looks OK, needle not scored or deformed in any obvious way. The mixture is not far off, judging from the way the engine starts and performs, but that is with the needle in as far as it will go.

Suggestions?

Thanks,

John