The specific-model manuals are OK for tuneups, but not for carburetor overhaul, or for helping you understand fully how the carburetor works, or for trouble-shooting. Start by downloading the Service Manual, which covers all Walbro diaphragm carburetors:
http://wem.walbro.com/distributors/servicemanuals/ServiceManual.pdf(An 8.8 MB manual must be better than a 1.3 MB manual - all those extra pages must tell you something useful. In this case, the real manual tells you how everything works in nauseating detail.) It also tells you how to remove the main nozzle and its check valve, but it seems fairly unlikely that is a problem area. Difficulties with the diaphragm carburetors seldom seem to stem from dirt that far inside the carburetor, perhaps because the dirt would be more likely to affect the high speed mixture adjustment. One of the most common problems after an overhaul, is either too much or too little fuel being supplied due to misadjustment of the metering lever: this is a critical adjustment.
You didn't ever explain whether the engine will start and run briefly if you put fuel in the spark plug hole, or the carburetor air intake. Also, you didn't explain whether the spark plug gets wet or stays dry whey you try to start it.
