Hi.
Carbs can be frustrating little buggers when they're not working properly!
Did you re-install the spring under the accelerator pump when the carb was apart? Check that the linkage is not binding.
As far as the choke 1/2 on and it running fine, check and make sure there are no vaccum leaks(try propane or carb spray) with the engine running, squirt carb cleaner all around the base, all vaccum lines, and throttle shaft. If you hear a difference in the way it runs especialy if it smooths out, you've found your problem.
I had trouble with a stock Hitachi once that I chased for quite a while-turned out to be a worn bore in the upper housing where the power valve pintle moves in!
That created an internal leak which was a bugger!
Hope any of this can help.
-Donnie

Gday having a bit of carby trouble b10 standard motor n carby
Just replaced the pump plunger would not return, ran good for while but now sounds like its flooding on acceleration, will run really well with the choke half on. Not to good with carbys any ideas