The International Cessna^® 170 Association

The back end of the right side muffler blew out during flight on my 55 170B, O-300. The engine sometimes backfires when I pull the throttle back during taxi and in flight. I don't know the last time the muffler was replaced, but is the increased pressure caused by backfire enough to blow out the back of the average muffler?

Thanks, ToolHead 4424F

Well your engine should not be backfiring nor should your muffler be falling apart. You muffler may have had a crack in the weld that may not have been detected coupled with likely an over rich mixture; have a close look at it and determine if it was due to corrosion and it was simply the mufflers time or whether a new bead on it can repair it.
No problems with setting up the idle mixture? And has it been awhile since the carb has been overhauded ?

Backfires can be caused by an intake leak

I believe a backfire is through the intake. An afterfire is through the exhaust and yes it can blow your muffler apart. It is basically a small explosion in your exhaust due to fuel in the exhaust. I had this problem a few times on my plane while reducing power. It was due to an exhaust valve sticking.

Thanks for your input Fred. I've have two 170 owners tell me the O-300 exhaust valves stick. Not if, but when.

ToolHead

My '52 B model used to "Popety--pop--pop--pop" when I closed the throttle on the downwind to turn base, and I was known for it up at Johnson Creek. That was on the old O-300 engine with a worn out muffler. Sticking Valves on the O-300? Terrible problem! I kept looking for one but never found one in twenty years of operating what is probably the most dependable Continental engine ever built.

You do not need a backfire to lose the back of a muffler. the end of the muffler had a dome in it to help absorb the
flexing from heat expansion, the radial weld can develop a crack and fail without a backfire. It happened to me in a cessna 140 which has the same style mufflers, and bellive me that will get your attenition.

I'm with bluEldr on this one. In the thousands of hours of operation of Continentals I've had only one stuck valve and that was while flying someone ELSE's airplane which was a maintenance nightmare. I've never had a stuck valve in my own airplane (knock on wood).

An explosion in your muffler means you've likely had fuel-rich gases collect there...then an open exhaust valve allowed a flame to set it off.

Do not "yank" your throttle back to idle. Reduce power judiciously... and plan your descents/patterns so as to avoid long idling descents in flight. (And visit with an experienced instructor about the hazards of idling with carb heat applied for long periods inflight, which create this rich condition that allows muffler explosions.)