Old Kettler guide pulley

[BSA]

My dad has just purchased an old Kettler 6 speed bike. It seems in good nick apart from when changing gears under load the chain seems to fall off the guide pulley.

The teeth are worn a little but not excessively what should I look at to stop this happening?

Could the chain be too loose? or the derailleur bent slightly? Any ideas, starting points appreciated.

 

[Globalti]

What's a guide pulley? Do you mean on the derailleur?

If so the pulley is worn and needs replacing. At the same time consider replacing the chain and the rest of the transmission.

 

[BSA]

What's a guide pulley? Do you mean on the derailleur?

If so the pulley is worn and needs replacing. At the same time consider replacing the chain and the rest of the transmission.

I mean the top pulley on the rear derailleur. I thought it was called the guide pulley.

Isnt replacing the derailleur, chain and transmission a little dramatic. Is there nothing else I can check adjust first?