You guys will probably kick me off this list for asking this question.  But
what happens during a backfire?

Dave Leonard

A loud bang! :-)

Paul Lamar

Dave,

In some backfires, exhaust containing substantial unburned fuel
exits the engine and travels down the exhaust system.  It apparently
cannot burn due to an absence of oxygen.  When this very hot mixture
reaches oxygen, it detonates, at a leak or the end of the exhaust.  An
alternative case is a flammable mixture (fuel and air) that travels
silently until it reaches an ignition source.  I imagine that the second
case is what is happening here.

Probably due to shutting off the spark and the engine continuing
to pull fuel-air mixture thru the carb as it spins down.  The unburned
fuel-air mix hits the hot exhaust system and "BANG".  Probably
need to cut off the fuel to shutdown, like a regular A/C carb.

Bill

 
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