I have been having problems with an Irvine 53 engine that overheats on midrange.
(See jbover, Irvine 53 problem 4/10/04 in the glow engine forum)
Using Slimline pitts muffler, 11X5 prop, 10% nitro wildcat fuel, GP RV-4 plane, Perry Pump. beneath cowel.

Bench tested today with Stock Muffler, Ran great, no overheating at midrange, left carb settings as is, replaced stock muffler with pitts muffler, Overheated at midrange.
This Pitts Muffler is rope-a-dopin me, but why?
Jbover in the pitts.

Common ailment with the Pitts style muffler, not enough back pressure ergo not enough tank pressure or Perry pump not pumping enough volume.

Try closing off one of the exhaust holes on the pitts about half way and see if it improves. Close it more if necessary, you may need to close one hole almost all the way on the pitts muffler. Many people have had to do that, you can squeeze it in a vice or heat it some and squeeze with pliers.

Wouldn't it be cool if they sold a valve that you could adjust that would fit inside one of the exhaust holes. You could tweak it until it was perfect back pressure. Of course it would have to be cheap and weigh nothing.

2 things about the causes you have with this engine spring to mind
1. Drop the nitro to 5% - UK engine, timed to 0-5% nitro, you will have pre detonation causing overheating with anything higher.
2. Increase the size of your prop - an 11x5 is way too small for the Irvine .53