Header,teflon, Muffler?

[BUDDYonRC at aol.com]

I have been having occasional problems with RPM and what seems like 
overheating on my YS-140-L. I have come to believe the problem is restriction in the 
muffler, but the muffler seems to be open and OK
The problem seems to have something to do with the prop size also. 
Running a 15-1/2 x 13N rpm at about 8200 everything is fine, switched to a 16 
x11 and running @ 8400 rpm and later in the flight the power back's off, I 
think it is heating up.
Yesterday I switched to a 16 x10 prop turning 8800 rpm and half way through 
the flight I heard a loud pop, After that the engine would not turn up. I 
landed and found that the Teflon coupler had split long ways about three inches 
long and had curled inward restricting the flow. This has happened on two 
different mufflers a Hatori and a Johnson that is shaped like the Hatori Has anyone 
run into this problem before? and what is the solution?  Is there another 
Aluminum muffler that will work? I have another setup using a CF muffler and have 
not had problems with it even at 9000rpm with a 16 x11 prop but choke when I 
think of spending $175.00 for another muffler.
Buddy    
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