To all of you knowledgeable people; You are correct (of course) - I now think the problem must be the "nozzle check valve" referred to by JD
My apologies for persisting with the idea that there must be a valve/reducer on the pulse system. Mine looks quite different to the above (JD supplied photo) being a brass fixture with a hole/nozzle facing the inlet side of the combustion chamber. I agree, this is a poor excuse for not taking your advice sooner.
The question now is;
- Can this be repaired?
- Replaced?
If the answer to the above is no - what is the solution(s)