Chuff problem

if it is a puffer type smoke unit the pivot point on the puffer arm may be worn. this puffer arm also triggers the cherry switch. the pivot is a split circle, squeezing the circle closed a bit with a needle nose pliers should fix the problem. if it is not a puffer unit then this will not be the problem.

It's a fan driven smoke unit in this one.  I'd be looking at the chuff switch, perhaps a slight bend of the arm will bring the extra chuff back.

The cherry switch in this engine is located between the third set of drive wheels.

This is what it looks like