here's what i get.. but i'm not at all sure about it.. and if someone has a better theory, jump right in.. the water's fine.
Originally, the carb had no airbox and no filter. Instead of installing a box with a tiny intake hole like a stock one, just a wide open filter was installed. So, the bike was running real lean and died at half throttle.
Next, the needle was removed. Although there is some metering below half-throttle, anywhere above that and the engine floods and dies.
So it went from dying due to leaness to dying due to richness.
Taping over the intake may have helped before the needle was removed, but at this point it needs more air than can be provided by any means..
The best that can happen without a needle (assuming enough air were somehow provided) is it might be made to run near full throttle but not below.. kinda like a model airplane engine.. but I wouldn't know how to accomplish that.