backfire through SU and rough running

[Austin in the Shed]

You don't say the spec of your engine,so guesswork.ie
Late Ruby heads tend to oil plugs.
Bosch "type" distributors can fail inside,condensors fail.Arcing though rotor arm (general)
Is it a good spark you have ? What fuel supply,pump ?If so is it pumping correctly.Blocked carburettor jet ?
If you are trialling seriously you need to keep most of these bits as spares.
Unlikely the valve timing has moved.More likely fuel supply or weak spark I think.