For a better understanding of the carby, see the sticky post "tuning the walbro" at http://www.prme.nl/forum/viewforum.php?f=7
I have linked to manuals etc. so you can get a good impression on the inner construction, and how to adjust the carb properly.
For cleaning a carb, WD40 is very good. This is sold with a thin spout, that can reach into the carb. I have noticed "roaming debris" in several weed appliance carbs that I serviced, so I would not exclude that possibility. This kind of debris in the carb can drive you mad, because one moment it runs well, and the a few minutes later it will not tune at all.

In your case, the pressure in the fuselage obviously is lower than the in cowl pressure when the engine runs. I think that the cowl pressure is higher than atmospheric, so the engine should tune well with the pressure line in the fuselage. The higher cowl pressure helps the carb lean condition. This should be cured first.
Take the carb apart, and clean it well. You may even have to clean it several times.