On a 35 oz. profile (CG shoestring) with fox 35 (stock) how much fuel is needed for a 6 min run, or enough to do the beginner pattern? I can only fit a 4oz. to this plane after I lengthened the nose a bit, the plans called for a 3 oz. or less. fuel will be 10%nitro 25% oil.

You'll be fine with the 4 oz. tank, and probably need only 3½ oz. For the full AMA pattern, on 10% Nitro, you need a bit more than 4 oz., but for the Beginner Pattern, you're OK. Incidentally, you can probably squeeze out a full AMA Pattern by reducing the Nitro.

Not having used a Fox Stunt on a competition quality airframe since mufflers became a part of the event, I didn't know they could affect the fuel economy so much.  I used 3 1/2 Ounces for seven minutes, pre-mufflers, for 500 to 560 square inch planes, on 10% fuel.  I relegated my old Foxes to sport C/L after the Max-S 35 (and Tigre G21-46) became available, but keep on running them occasionally for fun. 

On small airframes, such as Goldberg Shoestrings, even with mufflers, and not having taken stop watches into the center of the circle with such for-fun birds (ABC 35), I really didn't notice much fuel economy change from large airframe to medium, or muffler vs. none, running 10%.  It surprises me to hear that 4 Ounces might not be enough. 

Heresy or not, Jim, if it works for you, who can knock it! (I still prefer plenty of castor in a Fox 35 - that's just me...)

Gordon,

if you can fit a 3.5 or 4 fl oz tank, do so with whichever you prefer. Time how long a full tank flies for you. If it is too long, graduated syringes (the ones with volume markings on the side) make it simple to fill to a measured amount. You can either fill the tank to overflowing, then pull out 1/4 or 1/2 fl oz or so, or check how much your tank actually takes to fill, and fill to the smaller amount that gets you thjrough your flight time.

If you need to visualize this: Say you have a 5 fl oz size syringe. Fill it to the 5 oz line, fill the tank until it starts to overflow. What's left in the syringe plus what's in the tank is that 5 fl oz you started with. If there's 1.5 fl oz still in the syringe, the tank is holding 3.5 fl oz.

You could start filling by drawing 3.5 fl oz into the syringe, with mebbe a bit more to make sure.

At the other end of the flight, if it ran 7 minutes every time, you're getting a consistent 2 minutes per fluid ounce. That's one minute for a half fluid ounce. So, if you want 6 minutes, fill the syringe half a fl oz less... (These numbers are just examples - your results may vary.) Don't sweat being exact at this, so long as you have enough to complete the flight you want to make.