I thought I would add,
The best runs I have ever gotten from a Fox is with the fuel recommended by Larry Foster for his L&J Fox 35's. Excaliber mixed the fuel up, it's 15% nitro, 25 % castor and 3% Klotz. This stuff worked like a charm. Good, steady runs, reliable breaks and good starts. I used three Fox 35's from various drawers and flight box cubby holes that hadn't run in years. I used these with Michael, my 12 year old, in his Primary Force and they all ran the same even though one was way more powerful than the other due to it being New Old Stock (no muffler lugs, how old? I have no idea). It was using another new-to-me innovation, a super simple clunk tank on ram air uniflow with vents as described in the Primary Force assembly book, that provided the fuel to these Fox 35's.
I put this out as another avenue to running the old Fox that for, made it a viable Stunt engine for smaller models.
Chris...