Delayed Backfire

[Ron Lachniet]

I finished my 31 tudor a couple weeks ago and it seems to be running real good but every once in a while it backfires about 10 seconds after I turn it off. What causes a delayed backfire.

[Gary WA]

Unburned fuel in the carb,what I do is turn the fuel off and let it run till it stalls on it's own.

[Tom Wesenberg]

My Cub Cadet 149 did exactly the same thing. Without fail about 5 to 10 seconds after shutdown it would let out a shotgun blast. Then all of a sudden last year it quit doing it. I don't know why it quit, but I thought it was funny to hear the blast.

[Mike V. Florida]

Timing change fixed my car when that happened to me.

[roccaas]

Switch out the condenser.

[steve s]

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Originally Posted by Ron Lachniet

I finished my 31 tudor a couple weeks ago and it seems to be running real good but every once in a while it backfires about 10 seconds after I turn it off. What causes a delayed backfire.

Probably not the carb. Probably backfiring out of the exhaust system. Something is happening in the exhaust system to set off the unburned fuel that gets pumped out with the exhaust. My guess is a leak at the muffler clamp connection--as it starts to cool down, oxygen rich air is drawn in and conditions become right for ignition. Something also may be wrong that too much unburned fuel is getting out of the combustion chamber. This is commonly due, paradoxically, to too lean a mixture to support combustion so the whole mix fails to ignite in the head and is swept on out. Any of a million electrical issues can also cause unburned gas to be swept into exhaust. Both of those normally lead to backfiring WHEN running. The former when backing down from speed or changing gears; the latter under load. If neither of those happen and the car runs really well normally, with no missing, I'll go with the exhaust clamp leak possibly supplemented with too rich a mixture. Should be worse after a long hot run; not so bad if you shut down after a cool-down idle in the driveway.

Steve

[Ron Lachniet]

I think you are right, it is coming from the exhaust and it only happens about every fifth time I turn it off. I will check for an exhaust leak. Thank you very much.

[40 Deluxe]

It CANNOT be ignition related because the ignition is off. First, there has to be something in the exhaust system glowing hot enough to ignite fuel, and then there has to be unburned fuel vapors getting in contact with this hot spot, along with some oxygen.
After you kill the ignition, as the engine coasts to a stop it is still drawing in fuel from the carb so the exhaust system gets a dose of these vapors (now mixed with fresh air) which hit that hot spot, and POW! At the fleet where I work we have several truck-mounted air compressors with air-cooled V-twin engines. Due to emissions regulations, they run lean with retarded ignition timing so the exhaust system gets extremely hot! Without fail, when they are shut off, there is a loud backfire after a few seconds. This even after a few minutes of idling.

[Ray64]

Always idle down before turning the ignition off.If engine is to fast when you turn the key off it will suck extra fuel into the engine and then into the hot exaust causing late explosion in the hot muffler.This was my problem.Some people rev the motor just before shutdown. RAY

[Rowdy]

Carbon build up in the combustion chamer can caise this also. Beings it is not happening all of the time likely to be only one cyl, that happens to stio with the exhaust valve open and abit of unburned fuel. It may be abit too rich for heat detonation until air can get up the exhaust pipe and into the cyl. Thats my guesse. Rod

[Jerry Parr WI]

Steve S is correct. You have an exhaust leak. Check for a cracked exhaust manifold. When you turn off a running engine it pumps unburned fuel into the exhaust system. As the exhaust cools it contracts and draws fresh air thru the leak until the mixture is just right for the hot exhaust parts to ignite it.