Nice bike BTW. I have a few questions to ask you (which may just help you solve the problem yourself):
1. Have you used starter fluid in spray can form to verify whether you have any Intake leaks. You might be sucking air through a place other than your carburator. If that's the problem, wise men on this forum can explain in better detail than I can how to find out, and how to fix. I rather like Form-a-gasket. But don't do anything until you're sure.
2. Have you taken all the slack out of your throttle cable? Might help you there.
3. I kinda doubt it's a fouled plug, but that might be a contributory factor. Might want to check that too.
I may think of other things to ask later but for now that's all I got.