Ralph,
This carb looks as if:
1) the main needle is capped off (not accessable) to prevent user tampering (CARB regulations) (BTW: CARB = california clean air recommendation board or something like that), in order to set the mixture predefined for one single type of engine, for one single type of use.
2) fitted with a non-adjustable jet, again to obey CARB recommendations on CO emissions

so only the low needle can be adjusted.
Since we all know, i hope, that the low needle has influence from idle way up to full bore running, that one needle can make or break things. Set it too lean, and throttle response will suffer, and high end will be too lean. Set it rich, and low end. midrange will suffer, but high will be OK for us, not for the CARB Marshalls.
Midrange will putter alang as soon as the engine stabilizes, but this will not do well for 3D folks and for those that want their engine to respond crisply irrespective of the amount of running time at reduced loads.
These carbs suck, and veer past their target intentions ( if you want my opinion, in model engine application, they miss the target entirely!)