I have an Irvine 53 glow engine problem that I have posted on this forum and recieved many responses to my post. Thank you everyone who responded.
The problem was that the engine would run fine on the bench with the stock muffler, but I installed a Slimline Pitts muffler so it would fit in the cowel.
I would overheat and die during midrange, also could not make it run rich enough so I installed a Perry fuel pump, plenty of fuel, but still overheated.
At the advise of several respondants, I plugged one of the pipes on the Pitts and Eureka!
Problem solved, runs great with or without the pump.
I now believe that when there is not enough back pressure, the incoming Fuel/air mix is lost out the exhaust and is leaned out in the cylinder, causing it to overheat, plugging one pipe of the pitts gave it the right amount.
Could have fixed the problem for $1.17 from the git go. I pays to read R/C Universe!

Good find. Some of the MVVS engines have the same problem. My MVVS .77 likes the back pressure as well.
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Glad you're running well now. It just goes to show you how the Bisson and Slimline Pitts muffler makers do not give any thought or attention to what the correct exhaust opening should be. It's a typical problem with the Pitts mufflers. Don't the makers realize that when they design a muffler with basically double the exhaust opening outlet area compared to stock factory muffler that the engine wont perform properly. They only shape and build the muffler but basically don't know, understand or appreciate the details of nitro engine operation. At least the info you needed was here to be found.