Is this a new engine or have you run it in the past with no problems and are now trying to use it after a layup? If it is a new engine, I had a similar problem with one of mine, in that the manifold had a small hole in it and was leaking diaphram pressure that should have been going to the carb. Pull the little plastic manifold and check to see if you have the holes lined up. If you do, check the grove that sometimes allows you to turn the carb around 180 degrees. That little grove was perferated on my manifold. A replacement and the engine ran fine.
Check it out.