Why Does My Plane Backfire

[docket]

Today my Eagle backfired (or popped) one time when I was starting up.  She had been sitting on the ramp for about 8 hours.  The plane started well -- almost on the first rotation.  Don Maxwell did mags on her at the last annual so presumably its not a spark issue.  What causes a backfire?  In my car days I usually thought it was a timing issue but the plane seems to be performing just fine.  I climbed out of Dallas this afternoon at full power all the way to 8000 without a hiccup.

[Shadrach]

This was probably technically "after fire" in that it came from the exhaust as opposed to "back firing" through the intake...  Sometimes on start up, part of initial prime (especially if one over does it a bit) ends up getting sucked into the exhaust and ignited, causing a small explosion or "pop" sound in the exhaust. This was probably no big deal...  

As an aside - many years ago, I saw (ahem) "someone" inadvertently turn the key to far to the left on a mag check. When the the key went to the off position, the mags naturally stopped producing spark. Realizing his error quickly the youg pilot moved the switch back to both. The resulting boom sounded like a shot gun going off and garnered much attention from the coffee club sitting out side. Thankfully my...I mean "his", exhaust system survived the incident. It's not something we want to do often, but I am sure your little pop did nothing to hurt your exhaust.  

[DaV8or]

This kind of outer engine combustion can destroy your exhaust if severe enough. Many, many years ago I pressure washed the engine compartment on a '64 Falcon I had prior to doing some work on it. It took forever to start. During this process there was an enormous bang! After this, I took off the distibutor cap and indeed found moisture, so I dried it out. The car then started, but it now sounded like an angry motor boat. Inspection underneath revealed a completely ruptured muffler. Blown wide open. I don't pressure wash engines anymore.

My guess is, you primed it just a hair too much.