Why does rover 75 will not pick up speed until revs 4000 and then it shoots away?

Answer 1

If you have a V6 (2.0 or 2.5) then it is probably the VIS motors attached to the upper inlet manifold. They vary which air-plenum chambers are open according to revs and whether or not you are accelerating. One opens at 3000 revs and both at 4000 if I recall correctly. The motors often fail because they become oil-fouled or the micro-switches on the PCB board inside have come loose. They are repairable (see MG-Rover forum). Motors also fail if the butterfly valve or the slide actuator is stiff or seized. The butterfly valve (rearmost actuator) can be easily removed and, if you apply lots of patience, cleaned up. the other is more difficult and messy. The upper-manifold unit is bonded and cannot be taken apart to repair and a new one is very expensive.