Hello Mick,
My waterjets have the straighteners after the impellor behind the end piece if that makes sense, I still feel that you are getting trapped air in the one that "froths up the water " the impeller is cavitating and cannot clear it out .
The next time you try it in water and it froths lift the stern up slightly until the end of the jet is at the waters surface and see it it then clears the air out, you will soon know if it does as it will stop " gurgling " and drown you in a jet of water,, if so you need to make the rear end lighter to allow the jet to clear the trapped air automatically.
A couple of pice of my impellor and straightener , its off a Graupner Jet 2
Hope its of use buddy.
Sub