I'm no expert, but tend to read alot.
A guess without being there is either an air leak, meter arm heighth, or pop off pressure.

Here are two sites that have a boatload of good things on gas carbs and tuning.
[link=http://tech.flygsw.org/]Walbro Stuff[/link] Go to the Engine section, and then to the "Gas Carb Tech" link

Here there is lots of good info on tuning -> [link=http://www.geistware.com/rcmodeling/glowpower/index.htm]Geistware[/link]

And here is a link to a good thread on another board on tuning -> [link=http://www.flyinggiants.com/forums/fg68/427-gas-engine-tuning-newbies.html]Tuning[/link]

Get a coffee or a beer and spend some time. It's worth it

Good Luck
Bob