The inlet manifold is an obvious suspect, the VIS motors are known for behaving badly.
However I would firstly check the accelerator cable for tightness. Adjust so that it has little slack at the pivot point in the engine bay. If there is slack, it affects the downshift of the box a lot. If adjustment is needed, lift the plastic stopper out of its holder and adjust it by turning it on the thread the required way.
And, for good order, check that nothing obstructs the travel of the accelerator pedal, eg. carpet, rubber mat etc.
Good luck
|