Not to discount the potential for a leaking carb mount, but a bunch of us ran across a similar problem a few weeks ago, and there was a string specific to it that ran like 5 pages. In the end, it was discovered that the spring under the needle valve lever was too strong, overpowering the diaphram's ability to open the needle valve. If it was replaced by a spring from a Walbro rebuild kit you're back in business.

http://www.rcuniverse.com/forum/m_98...tm.htm#9983076

I was able to get mine going with the original spring by adjusting the lever VERY slightly to the point it was level with the carb body around it.

If your choke plate has a hole in it, you can make the engine dramaticaly easier to start by hand by simply soldering that hole closed. Mine starts easily within 5-10 flips cold, including the choking process.