sounds a bit like the fuelmaster thingy i have right here in my 1994 mag, made by Ram-air it takes exhaust pressure to regulate fuel supply to the carb, in these tests it worked with the tank 10" away and held nose up, nose down with no change in RPM. trying to find one but can't find anything on it. looks like a simple unit, diaphragm, inlets and outlets for exhaust and fuel. costs £25
looks PERFECT for 3D or for my magnum which never runs the same in the air as it does on the ground. can scan pics if needed