Quote:
Originally Posted by Jim Brierley
The backfire comes from unburned fuel in the exhaust system. Start with a good tune-up. It could be a fouled plug, rich fuel mixture, etc., etc., or even a leak somewhere in the exhaust system.
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Quote:
Originally Posted by john in illinois
I had it and it was an exhaust leak letting oxygen into exhaust after shut down.
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No doubt, unburned fuel exploding is a backfire. Unburned fuel can come from either engine running too rich OR too lean (because of ignition failure causing unburned fuel/air mixture to pass on thru to the muffler).
Two things are required for explosion: a proper mix of fuel and air, AND high enough temperature. SO, why would it explode only after shutdown?
Although thermal seep from the combustion chamber after shutdown causes the coolant temperature to go up a few degrees for a few minutes, I don't think this causes backfire; the exhaust valves are never as hot as when the motor is running, and the exhaust manifold only gets cooler after shutdown.
The only thing that makes sense for backfiring only AFTER shutdown is that a leak at the muffler clamp or the exhaust manifold gasket lets air be drawn INTO the muffler as it cools down after shutdown. As the magic fuel/air mix is reached while the system is still hot enough, BAM!
Anything that results in unburned fuel in the exhaust system and overheated exhaust system will aggravate the situation: improper tuning, improper carb setting, and retarded timing makes engine run hot. BUT without an air leak on cool down, your car may struggle but it won't backfire.