Here are some late additional comments, now that I've finally looked at the service manual.

First, here is how you tune the pilot screw:
[Linked Image]

The doctrine according to Honda, is you adjust it with the air cleaner housing removed.

Here is the doctrine on setting up the governor:
[Linked Image]

Incidentally one of the interesting things I noticed is that the GV150 has an oil pump. It might be a good idea to clean it out and check that its input and output are clear. You access it from the bottom of the engine - no need to open the crankcase.

With regard to the engine speed being hard to manage between the high speed position and the choke position, perhaps that is why Honda switched to a more elaborate system on the OHV engines, using a very strong detent on the speed control lever between "Fast" and "Choke", as well as a screw-adjustable speed stop on some models.