Hi all -
I have the older Fox 35 stunt and attached an Aero Products tongue muffler with a 10x6 prop. The engine quits early and I'm believing it's getting a bit too hot.
Yes, I have to use a muffler - field rules.
Yes, I'm running 5% nitro, 29% all castor fuel.
I saw posts recommending dropping to a prop  with lower lower loading like a 9x6, so I can try that.

Next I measured exhaust areas.
The stack on the Fox has 0.246 sq in of total open-face exhaust area. Granted, that's much larger than the actual exhaust area right where the piston actually opens up into the stack, but it's what I measured.

The Aero Products tongue muffler has 14 holes at 0.076" diameter (#48 drill just fits). That's only about 0.064 sq" of area, which is only 25% of the open face of the stack.

Whaddayallthink?
Drill out another 14 holes?
I suppose I could drill out enough holes to equal the area of the engines stack, but then it would just "look" like a muffler full of tons of holes.

Dan, no - I haven't run it without a muffler.
However...
I did run it with a lighter 9x6 wood prop and it ran well. I'll try a 10x5 as well.
Yes, this is a Randy Smith strap-on muffler (direct). "Strap-on" in that I wrapped a hose clamp around to the back of the ears. Works OK but a standard hose clamp is a bit clunky-looking.

           I use the same muffler and rather drill additional holes, Rather then drill more,  I open the stock holes up with the next size drill bit. I've gone up to 7/64" but it does start getting loud.

Fox 35 Stunt muffler for an engine with mounting lugs. Made of DuBro muffler stock and uses 4-40 mounting bolts. It has nine .093" diameter holes for a total area of .061 square inches and makes sufficient power for a Nobler flown in light breezes. I use the pressure fitting on this setup. Running a 10x5 prop. I use this exact same setup on a Fancy Pants with no issues. Both with 4-2 runs.

These two mufflers have about 18% less area than your example.

Based on this, it wouldn't appear that your muffler restriction is the entire cause of perceived overheating and incomplete runs. Therefore, I would not start drilling more holes or modifying it in any way. Yet.

But we need more information to go on. How well broken in is your engine? How big/heavy is your plane? What kind of rpm/setting are you trying to run? Have you tried higher nitro? How tight is your cowling, if any? Is the piston or cylinder varnished up? Did this run fine before and this is an evolving problem, or is this a new installation?