I am flying a Fox 35 on a Taipan 11 x 5 prop, releasing at 7,800. Peter Chinn (Aeromodeller 12/1967) says Foxes vibrate in the 7,000-9,000 range of RPM, but mine doesn't.  Peak HP is 0.45 at 11,500-12,000 RPM. He gives 9,700 to 10,300 on various 10 x 6 props. I'd suggest running your Fox nd let it warm up for a few seconds then tach it.  Set the needle where it is just breaking in and out of four cycle.  That should give you in the mid to high 9,000's.

Now .... Please don't laugh at this question !

What should be the launch RPM for a 'stock' Fox 35?
I've got a couple of these engines but I'm only now getting around to flying with them. I only came back to Stunt C/L a few years back and I've been concentrating on using my Enya and Tigre engines .

My Fox 35's are carefully run-in [ all castor based fuel ] and will use a [ recommended on this forum ]  25%/26% lube content made up half and half castor/synthetic. I'll be using Master 10x6 or Zinger wood 10x6 props.

Much obliged for any contributions ,

Robin [ ex-pat Brit in the Charente full of ex-pat Brits ]

     I can only guess (since a very strong engine needs more ground RPM and a weak engine needs less, and they vary A LOT, as do the propellors), but on 5% nitro, I would guess somewhere around 8100-8200 rpm. Note that unless you are using a TNC Tach, it probably doesn't matter because the readings are so random on the others that you can't compare from unit to unit.

     Most people set them by ear, anyway - with a 10-6 on a typical model I set it for 90% 2-stroke, 10% 4-stroke (just occasionally missing a stroke here and there), and then count on the unload to get it into a "just barely still 4-stroking" setting in level flight. From there, it's either too fast or too slow for the airplane, and adjust accordingly. We used to do "needle flights" every morning just to get it close, and that's probably still what I would do.

      Brett