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Re: Backfire

> JimA=> the exhaust is under postiive pressure and will almost never
> draw => air INTO itself.  
> These objections make a lot of sense to me, and the common
> explanation for exhaust pops has never satisfied me. I wonder what
> you (or anyone else here) think *does* cause this phenomenon. 
> In fact, I wonder every time I leave my house in my Bus with the
> exhaust leak. I live on a steep hill, so as soon as I leave the
> drive I'm engine-braking in first. The exhaust assembly isn't hot
> enough at that point to cause anything to detonate. It reliably
> gives me a rapid popping until I put on some load at the bottom of
> the hill. I don't like it.

On overrun the engine is just acting as an airpump.  Since the 
throttle plate is below the venturi, most of what gets pumped is air, 
but there is still a little gas.  The mixture is generally too lean 
to burn, however, so it passes through the engine and into the 
exhaust.  There the gas tends to condense, and get stored while the 
air obviously just keeps passing through.

Eventually, there is sufficient buildup of gas to form an explosive 
mixture, so it will pop if ignited.  This will happen if one cylinder 
combusts occasionally, emitting a hot flame front out its exhaust 
port.  If your bus does this, then I would GUESS that something is 
set a bit rich, because my impresson is that a carbed engine on 
overrun should always be too lean to burn.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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