I'll mix a little thought for you---
Change the springs( In a jet boat engines are easy to work on) now and run it for the month then take it down over the winter---I have seen engines in jet boats hammer all the valves up into the ports so bad from heat/detonation from pulling all that power/load at lower rpms---the loose springs could be a sign of that
And I do BBC, BBF and others also!!!!!

