Rick and Bern are correct about spill timing. My post was simply based on an observation several years ago where a guy had turned the delivery valve somewhat on his little Massey... they used the same basic pump. It ran like hell and knocked more than normal... and in the process of finding out what exactly he did I discoverd that one delivery valve had been turned... When I turned it back the knocking stoped. Engine ran as it should.
That said... it"s possible that the spill timing has been off on this one for many moons and that IS the source of the problem..