If none of the advice regarding issues with the carb helps, then it can actually be a result of the mods done to the engine.

I was reading on the RB MODS website about some of the history, and he/they(?) actually introduced a harmonic into the air pulses within the motor that would cause the engine to stall at a particular RPM. This was created by presseure waves rebounding on themselves in order to completely stop the fuel supply within the sleeve area.

I wouldn't suggest this is your problem until all other avenues of checking for problems and air leaks within your engine are exhausted, especially if the mods are done by a reputable supplier. They too would have worked all of this out by now.

Cheers,

Mike.