How long will your engine idle on the test stand?
What happens when you increase the throttle after a minute of idling or so?
How far is your tank from the engine and how is it plumbed?
Have you testing running the engine nose up and nose down?
How much fuel has been run through this engine?

The tuning advice is really good above, so I won't repeat it. One thing I'll add though is that when I'm setting up a new plane, I won't fly it until it will idle for at least a minute on the ground, transition without hesitation, and run wide open with the nose up without going over lean. That's with a hot engine after the 2-3 initial break in tanks. After that, future tuning is to adjust for weather and to lower the idle speed after about 10 tanks or so.