I am by no means an expert but would suggest calling Keith at BME. He is EXTREMELY helpful. Are you running fuel filters? It is possible the screen in the carb could have ingested some debris and is plugged up. Also, Keith reccomended I silver solder a piece of brass tubing over the atmospheric vent on the diaphragm cover and running a piece of fuel tubing inside of the fuse. It seems that if the carb is not completely isolated it will richen due to a false atmospheric pressure signal from prop blast or airspeed (maybe both). I did this and the difference in smoothness at low/mid throttle was unreal. He also recommended going with 40 or even 50:1 on the mixture to keep the engine from carboning up which will also cause it to run erratic. As I mentioned, these are just some ideas. If it is an ignition problem I cannot help.
Stick with the engine. Once you get it sorted out, it is a fine running and powerful motor. Good luck, cbk