I've found that the carb block needs to be sealed with a gasket type sealant. If you've pulled the carb and block, and put it back on without the sealant, this could be your problem. Locktite makes an "anaerobic" gasket maker which is what I've used and what I've been told 3W U.S. dealers use when assembling engines. I have two 3w-50's and both have to have the sealant in order to have adequate fuel pull.
Hope this helps.
Sam