A simple explanation that has been overlooked is when the R22 was rigged. Summer vs winter. Also, expect a lower end RRPM at lower weights, its normal. Solo R22 autos are a bad idea because it IS an unforgiving aircraft of errors, it wasn't designed to be a trainer, it just happens to be very economical to operate. Respect the limits of the aircraft and you will have no problems.
BTW: I love them!