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I reversed the Needle valve position believing it would bu buried under the exhaust, forgetting  the engine would be mounded in the inverted position!  Put it back. Also didn't realize the venturi (or carb)  is offset.  Now the needle valve extends a bit more from the fuselage. 

I ran a couple tanks of fuel through the engine, rich with a 10-6 prop. Small, I'll admit but did not want to put a large load on the engine. I now have a Rev-up 11-6 EW mounted. What do you guys think?  I was asking about a muffler a while back. Mostly wanted to get the exhaust, and oil residue outside of the aircraft.  Went diging through my stuff and found a universal stack. I think it's Dubro but forgot. Not marked. Has a spring like spiral wire  to create a bit of back pressure just downstream from the pressure tap.  It certainly does not quiet the engine any. It came with a strap that wrapped around the engine. I drilled and tapped it for a pair of 6-32 socket head screws. 

Yea, thanks . tiss a ' 80's ' one . No worries .

You can take a disc grinder to the stock muffle , and loose half the weight . Leave the round ' spot ' to take a pressure nipple. J B weld the rear cone on. cut tube to 1 mm so holes not oversize .

A lot more aerodynamic, but ya get a metallic echo from the pulse / flex = ringing noise .