Make sure they are really 6100's, the 6000 used all the same parts, 6100.100 motor, but it had the smaller 7210.202 transformer, this is only 11V, a 6100 was 19V and used the 7410.602 transformer. If you can find the bigger transformers that would double the flow. If the props are more than 5 years old, usually changing the drive unit for a new one will make a difference as the props have worn and the magnets have lost some power.