I don't think that is what is happening....
Don't get me wrong: I do not own one of these engines, so I am just "experimenting in the brain" here.
Fuel draw is not effected by vacuum in the carb throttle, but based on the bernouilli-effect (increase of local velocity is decrease in local pressure). This is the same effect that carries paint from an atmspheric container upwards to the nozzle of a paint gun, even if the air pressure is way over atmospheric.
The mixture strenght SHOULD be effected by the needles and the bernouilli effect, and the function of the regulator should be only to keep the pressure before the needle at a very constant level. The pump/regulator has exactly the same function as the float&bowl in a conventional carburettor: to maintain a very constant pressure regardless of fuel flow (the float does that by maintaining an exact liquid level, as liquid level=column pressure).
If that regulator cannot supply a constant pressure, than you will never get that engine to run good. it is just as simple as that. But if the pressure regulator is OK, even a glow-style carb should do the trick, maybe the needle profile needs to be adapted to the fuel (similar to for example Lötterle Carburettors having different needle shapes for every Nitro content or oil content).
For example look in this thread, where people have managed to run glow engines on gas, with just a glow carburettor: http://www.rcgroups.com/forums/showthread.php?t=1476259
So, a glow carb can
work with gas, it is just way more pressure sensitive....
I really think, that adjusting the location of the spraybar will influence the whole behaviour of the carb, but I think, that with adjustment of the spraybar location, both high and low speed needles need to be readjusted due the changed fuel draw.
Come to think of fuel pressure: with the big gassers, propwash entering the atmospheric hole of the fuel regulator has many times caused engines to run irregular. Can that be an issue with this engine with its loose fuel pressure regulator????