Just curious....Have you made sure that the fuel lines/tank are not leaking?
Do this: take the line from the vent, and pinch it off. Take the line to the carb and blow into it, pinch it off, hold them for a few seconds, Then release the vent line...should have a nice stream of fuel squirt out.
If thats ok, try blowing into the vent line. Be sure you have a nice stream coming out the line to the carb.
Also...I usually remove the carb and put a little sealent around the base and set it back in the crankcase.
If these tips dont help, lastley, id try getting a chunk of fuel line, and slip it over the needle valve onto the carb body....Its a cheap way to see if the oring on the needle valve is worn out. Also...When is the last time you tried a Glo Plug and new washer...
These are things off the top of my head, hope they work for you.