The usual source does not have a manual for a GV150, only for a GV100 or a G150. Some of the literature suggests it is the same carburetor. However, the GV100 illustration shows someone adjusting the pilot screw while the carburetor is on the engine, which doesn't sound as if it is on the back of the carburetor. Here is the GV100 download URL:
http://www.honda-engines-eu.com/en/engines/models_range/GV_range/GV100/shop.jsp

Here is the G150 download URL:
http://www.honda-engines-eu.com/en/images/1004.pdf
This one says just set the pilot screw to a standard 1 and 5/8 turns, and you're done. That sounds a lot more like a screw that you can't access when the engine is running.