True about China. It is simply hit or miss, but buying the BRANDED units seems to mean that there is some quality control involved. Recently I rebuilt the carb on my Stihl HL100 hedge trimmer and guess what. Carb said Made in China (Zama unit), the bowl-side cover said Stihl.
Good to know the primer bulb was the problem, and cheap and easy to replace. Sure not fun to get to that point, though.
It's difficult when throwing parts at a simple device and still doesn't correct the issue. I'm dealing with a Troy Bilt 4-choke TB4BP backpack blower that starts first pull even when dead cold and on half choke, idles perfectly, but bogs on initial throttle unless you very gingerly open it up, and won't stay at WOT - misses, chokes, wants to die. If you back off the WOT just a tad, it runs fine. Just doesn't seem like full RPM though either but maybe because it's spinning an impeller vs a simple trimmer head. I've replaced OEM MTD parts - spark plug, air filter, carb. Adjusted the valves, changed oil. It might be the fuel lines, but they look newer was told they were replaced about 2 years ago (this carb has 2 lines, one main and one return from the bulb built into the carb) although the vent gets into the fuel level when it's topped up - no avoiding it because of tank shape, similar to 4-choke trimmers too. Tried a known-good fuel cap from my other similar trimmer.
It has run the dang same before carb, after carb, before plug, after plug, before valves, after valves before filter, after filter - everything. There are no Lo/Hi adjust screws, just idle. So it's bugging me..... only thing I can think is the ignition module is bad or some incorrect spacing or rust on flywheel, but this thing is 10 years old and was really clean all over did not appear used and abused. So driving me nuts but I'm out of simple parts - other than the ignition module I guess which will be a pain to remove the whole impeller housing to get to it.