Adding weight to Farish B1

[Wingman mothergoose]

Is it possible to add some more weight to the loco body of the Farish B1? If so, where and how? I'm still having trouble with the driving wheels sticking in places during forward running, but I've found that if I balance a Peco track rubber on top of the loco body this eliminates the wheels locking up...
I've tried pretty much everything I can think of to free up the motion on this loco, have adjusted the eccentric rods a bit and this has cut the locking up right down, but it still does it, and I don't want to end up destroying the valve gear by bending or snapping one of the rods!
If there's anything I can use of an equivalent weight to the Peco track rubber I'd love to know how to add that weight to the inside of the loco.

Chris